This dataset includes a series of modified UK administrative boundary shapefiles based on the 2011 census which are intended for use in more accurate visualisation of UK geospatial data analysis. There are two key features of these shapefiles: (1) administrative shapes have been clipped to the Ordnance Survey buildings shapefile, so that in choropleth visualisations relating to demographic data filled spaces represent populated areas of the UK rather than large undifferentiated blocks. (2) Shapefiles have been simplified to reduce loading and processing time, in the case of this repository at 100m. After testing, we have settled on a procedure to render buildings layer visually comprehensible at high zoom levels, by adding a small buffer, dissolving (so that individual overlapping shapes combine into a single more easily visualised shape) and then simplifying at 150m. It is important to emphasise that because of the use of simplification (using a Ramer–Douglas–Peucker algorithm), these shapefiles are not suitable for analysis as boundaries may not be suitably precise or accurate. For users interested in the process used to generate these files you can consult the codebase deposited on github.

Many thanks to colleagues including Alasdair Rae for recommendations on technique used here. Computations were performed using the University of Birmingham's BEAR Cloud service, which provides flexible resource for intensive computational work to the University's research community. See http://www.birmingham.ac.uk/bear for more details. Given the massive size of datasets involved (including the district buildings vector shapefile which is 1.4gb and consists of hundreds of thousands of individual shapes), this work would have been impossible without this invaluable resource. I hope that these files will be of use to colleagues who may not have access to similar large computational arrays and make the process of visualising UK boundary and census data more accurate and efficient.

Original files are under OGLv3 licenses. Derived data files, where possible are licensed for use under CC BY 4.0.

Files include the following:

Original unmodified data:

infuse_ctry_2011.zip - original country level shapes, based on 2011 census, downloaded from https://borders.ukdataservice.ac.uk/ukborders/easy_download

infuse_dist_lyr_2011.zip - original local authority shapes, based on 2011 census, downloaded from https://borders.ukdataservice.ac.uk/ukborders/easy_download

TermsAndConditions.html - UK Data Service license details (OGLv3), applies to all the above

GB_Postcodes.zip - UK postcode district shapes, prepared by Addy Pope, https://datashare.ed.ac.uk/handle/10283/2597

Derived data files:

OS_Open_Zoomstack_district_buildings.zip - buildings layer extracted from Ordnance Survey Zoomstack package, licensed under OGLv3 and exported to gpkg format.

*_simplified_100m.gpkg - Administrative shapes from above, simplified in R at a resolution of 100 metres.

*_simplified_100m_buildings_overlay_simplified.gpkg - Administrative shapes from above, simplified in R at a resolution of 100 metres, and then clipped to the buildings layer.

*_simplified_100m_buildings_overlay_simplified.gpkg - Administrative shapes from above, simplified in R at a resolution of 100 metres, and then run against the buildings layer as a difference layer. Suitable for using as an overlay as the shapes are inverse.

Users who wish to use these shapefiles in a reproducible research context may want to download individual files directly from this repository. To do so, you could use the following R code:

load packages

require(sf) # load simplefeature data class, supercedes sp() and used for st_read

given the size and complexity even of simplified files here, ragg is highly recommended

for users on macos given inefficiencies in default R graphics device

require(ragg)

create paths as needed

if (dir.exists("data") == FALSE) { dir.create("data") }

download data files only if they aren't already present

if (file.exists("data/infuse_dist_lyr_2011.shp") == FALSE) { download.file("https://borders.ukdataservice.ac.uk/ukborders/easy_download/prebuilt/shape/infuse_dist_lyr_2011.zip", destfile = "data/infuse_dist_lyr_2011.zip") unzip("infuse_dist_lyr_2011.zip", exdir = "data")} local_authorities <- st_read("data/infuse_dist_lyr_2011.shp")

Download high-quality, up-to-date United Kingdom shapefile boundaries (SHP, projection system SRID 4326). Our United Kingdom Shapefile Database offers comprehensive boundary data for spatial analysis, including administrative areas and geographic boundaries. This dataset contains accurate and up-to-date information on all administrative divisions, zip codes, cities, and geographic boundaries, making it an invaluable resource for various applications such as geographic analysis, map and visualization, reporting and business intelligence (BI), master data management, logistics and supply chain management, and sales and marketing. Our location data packages are available in various formats, including Shapefile, GeoJSON, KML, ASC, DAT, CSV, and GML, optimized for seamless integration with popular systems like Esri ArcGIS, Snowflake, QGIS, and more. Companies choose our location databases for their enterprise-grade service, reduction in integration time and cost by 30%, and weekly updates to ensure the highest quality.

This file contains the digital vector boundaries for Counties and Unitary Authorities in the United Kingdom, as at December 2019. The boundaries available are: (BUC) Ultra Generalised (500m) - clipped to the coastline (Mean High Water mark). Contains both Ordnance Survey and ONS Intellectual Property Rights. Download File SizesUltra Generalised (500m) - clipped to the coastline (200 KB)Units for the following fields:St_length = metresSt_area = metres2REST URL of ArcGIS for INSPIRE View Service https://ons-inspire.esriuk.com/arcgis/rest/services/Administrative_Boundaries/Counties_and_Unitary_Authorities_December_2019_Boundaries_UK_BUC2/MapServer/exts/InspireView REST URL of ArcGIS for INSPIRE Feature Download Service https://ons-inspire.esriuk.com/arcgis/rest/services/Administrative_Boundaries/Counties_and_Unitary_Authorities_December_2019_Boundaries_UK_BUC2/MapServer/exts/InspireFeatureDownload REST URL of Feature Access Service https://ons-inspire.esriuk.com/arcgis/rest/services/Administrative_Boundaries/Counties_and_Unitary_Authorities_December_2019_Boundaries_UK_BUC2/FeatureServer

Data OriginThe dataset provided by Ofwat is rooted in legal records. The dataset is digitised from the official appointments of companies as water and sewage undertakers, which include legally binding documents and maps. These documents establish the specific geographic areas each water company is responsible for. The dataset was sourced from Constituency information: Water companiesData TriageAnonymisation is not required for this dataset, since the data is publicly available and focuses on geographical boundaries of water companies rather than individual or sensitive information. The shapefile serves a specific purpose related to geospatial analysis and regulatory compliance, offering transparent information about the service areas of different water companies as designated by Ofwat.Further ReadingBelow is a curated selection of links for additional reading, which provide a deeper understanding of the water company boundaries datasetOfwat (The Water Services Regulation Authority): As the regulatory body for water and wastewater services in England and Wales, Ofwat's website is a primary source for detailed information about the water industry, including company boundaries.Data.gov.uk: This site provides access to national datasets, including the Water Resource Zone GIS Data (WRMP19), which covers all water resource zones in England. This dataset is crucial for understanding geographical boundaries related to water management.Water UK: As a trade body representing UK water and wastewater service providers, Water UK's website offers insights into the industry's workings, including aspects related to geographical boundaries.Specifications and CaveatsWhen compiling the dataset, the following specifications and caveats were made:This shapefile is intended solely for geospatial analysis. The authoritative legal delineation of areas is maintained in the maps and additional details specified in the official appointments of companies as water and/or sewerage undertakers, along with any alterations to their areas.The shapefile does not encompass data on any structures or properties that, despite being outside the designated boundary, are included in the area, or those within the boundary yet excluded from the area.In terms of geospatial analysis and visual representation, the Mean High Water Line has been utilized to define any boundary extending into the sea, though it's more probable that the actual boundary aligns with the low water mark. Furthermore, islands that are incorporated into the area might not be included in this representation.Ofwat’s data was last updated on 25th May 2022Contact Details If you have a query about this dataset, please email foi@ofwat.gov.uk

Regions were built up of complete counties/unitary authorities so, although they were subject to change, they always reflected administrative boundaries as at the end of the previous year.

The Region area list contains nine areas for English Regions, and provides coverage of England only.

Please visit ONS Beginner's Guide to UK Geography for more info.

The boundaries are available as either extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands) or

clipped to the coastline (Mean High Water mark).

This file contains the digital vector boundaries for Output Areas in England and Wales, as at 21 March 2021.The boundaries available are: (BFE) Full resolution - extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands).Contains both Ordnance Survey and ONS Intellectual Property Rights.File size: 1418 MBVersion amendment 14th July 2023: Additional Long/Lat/BNG_E/BNG_N coordinates and LSOA NM/CDs added. Overlapping intersection repair maintenance.

REST URL of Feature Access Service – https://services1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/rest/services/Output_Areas_2021_EW_BFE_V9/FeatureServerREST URL of WFS Server –https://dservices1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/services/Output_Areas_2021_EW_BFE_V9/WFSServer?service=wfs&request=getcapabilitiesREST URL of Map Server –https://services1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/rest/services/Output_Areas_2021_EW_BFE_V9/MapServer

Download high-quality, up-to-date British Virgin Islands shapefile boundaries (SHP, projection system SRID 4326). Our British Virgin Islands Shapefile Database offers comprehensive boundary data for spatial analysis, including administrative areas and geographic boundaries. This dataset contains accurate and up-to-date information on all administrative divisions, zip codes, cities, and geographic boundaries, making it an invaluable resource for various applications such as geographic analysis, map and visualization, reporting and business intelligence (BI), master data management, logistics and supply chain management, and sales and marketing. Our location data packages are available in various formats, including Shapefile, GeoJSON, KML, ASC, DAT, CSV, and GML, optimized for seamless integration with popular systems like Esri ArcGIS, Snowflake, QGIS, and more. Companies choose our location databases for their enterprise-grade service, reduction in integration time and cost by 30%, and weekly updates to ensure the highest quality.

The current counties of England are defined by the ceremonial counties, a collective name for the county areas to which are appointed a Lord Lieutenant. The office of Lord Lieutenant was created in the reign of Henry VIII. The Lord Lieutenant is the chief officer of the county and representative of the Crown. Whenever the Queen visits an area she will be accompanied by the Lord Lieutenant of that area. Legally the ceremonial counties are defined by the Lieutenancies Act 1997 as ‘Counties and areas for the purposes of the lieutenancies in Great Britain’ with reference to the areas used for local government.

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https://www.ordnancesurvey.co.uk/products/boundary-line#technical

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Source:

https://osdatahub.os.uk/downloads/open/BoundaryLine

Licence:

Contains public sector information licensed under the Open Government Licence v3.0.

https://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/

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This file contains the digital vector boundaries for Nomenclature of Territorial Units for Statistics Level 2, in the United Kingdom, as at January 2018. The boundaries are full resolution - extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands). Contains both Ordnance Survey and ONS Intellectual Property Rights.REST URL of ArcGIS for INSPIRE View Service – https://services1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/rest/services/NUTS_Level_2_(January_2018)_FEB_in_the_United_Kingdom/MapServerREST URL of ArcGIS for INSPIRE Feature DownloadService – https://dservices1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/services/NUTS_Level_2_January_2018_Full_Extent_Boundaries_in_the_United_Kingdom/WFSServer?service=wfs&request=getcapabilitiesREST URL of Feature Access Service – https://services1.arcgis.com/ESMARspQHYMw9BZ9/arcgis/rest/services/NUTS_Level_2_January_2018_FEB_in_the_United_Kingdom_2022/FeatureServer

Counties were formerly administrative units across the whole of the UK. Due to various administrative restructurings, however, the only administrative areas still referred to as 'counties' are the Non-Metropolitan Districts of England. The English Metropolitan Districts, although no longer administrative units, are also used for statistical purposes.

The Counties area list contains 35 areas of the following constituent English geographies:

• 27 Shire counties
• 6 Metropolitan counties
• Inner London and Outer London

Please visit ONS Beginner's Guide to UK Geography for more info.

The boundaries are available as either extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands) or

clipped to the coastline (Mean High Water mark).

Public Facing Administrative Boundaries set at 1:10,000 scale. Public Face boundaries are attributed with standardised names and codes for each area. This dataset is for Environment Agency and Natural England Public Face Areas.

Please note: This is a large data product with 2.7 million polygon features (1.2GB file in ESRI File Geodatabase format). It is not possible to download in Shapefile format. Please access the data using the APIs or select another download format.This is the spatial framework around which the Living England Phase II habitat classification is based. The segmentation was created in the Trimble eCognition software using Sentinel-2 Analysis Ready Data (ARD) image mosaics for winter (February 2019) and summer (June 2019).

Sentinel-2 Analysis Ready Data (ARD) produced by the Earth Observation Data Service (JNCC / DEFRA) were used as the input for the segmentation. The Sentinel-2 ARD is available under an Open Government License (OGL). It is not intended that the 2019 segmentation will be revised, however, as Living England progresses and up-to-date image mosaics are created new habitat segmentation datasets will be developed from the up-to-date imagery.Full metadata can be viewed on data.gov.uk.

PLEASE NOTE: This data product is not available in Shapefile format or KML at https://naturalengland-defra.opendata.arcgis.com/datasets/Defra::living-england-habitat-map-phase-4/about, as the data exceeds the limits of these formats. Please select an alternative download format.This data product is also available for download in multiple formats via the Defra Data Services Platform at https://environment.data.gov.uk/explore/4aa716ce-f6af-454c-8ba2-833ebc1bde96?download=true.The Living England project, led by Natural England, is a multi-year programme delivering a satellite-derived national habitat layer in support of the Environmental Land Management (ELM) System and the Natural Capital and Ecosystem Assessment (NCEA) Pilot. The project uses a machine learning approach to image classification, developed under the Defra Living Maps project (SD1705 – Kilcoyne et al., 2017). The method first clusters homogeneous areas of habitat into segments, then assigns each segment to a defined list of habitat classes using Random Forest (a machine learning algorithm). The habitat probability map displays modelled likely broad habitat classifications, trained on field surveys and earth observation data from 2021 as well as historic data layers. This map is an output from Phase IV of the Living England project, with future work in Phase V (2022-23) intending to standardise the methodology and Phase VI (2023-24) to implement the agreed standardised methods.The Living England habitat probability map will provide high-accuracy, spatially consistent data for a range of Defra policy delivery needs (e.g. 25YEP indicators and Environment Bill target reporting Natural capital accounting, Nature Strategy, ELM) as well as external users. As a probability map, it allows the extrapolation of data to areas that we do not have data. These data will also support better local and national decision making, policy development and evaluation, especially in areas where other forms of evidence are unavailable. Process Description: A number of data layers are used to inform the model to provide a habitat probability map of England. The main sources layers are Sentinel-2 and Sentinel-1 satellite data from the ESA Copericus programme. Additional datasets were incorporated into the model (as detailed below) to aid the segmentation and classification of specific habitat classes. Datasets used:Agri-Environment Higher Level Stewardship (HLS) Monitoring, British Geological Survey Bedrock Mapping 1:50k, Coastal Dune Geomatics Mapping Ground Truthing, Crop Map of England (RPA), Dark Peak Bog State Survey, Desktop Validation and Manual Points, EA Integrated Height Model 10m, EA Saltmarsh Zonation and Extent, Field Unit NEFU, Living England Collector App NEFU/EES, Long Term Monitoring Network (LTMN), Lowland Heathland Survey, National Forest Inventory (NFI), National Grassland Survey, National Plant Monitoring Scheme, NEFU Surveys, Northumberland Border Mires, OS Vector Map District , Priority Habitats Inventory (PHI) B Button, European Space Agency (ESA) Sentinel-1 and Sentinel-2 , Space2 Eye Lens: Ainsdale NNR, Space2 Eye Lens: State of the Bog Bowland Survey, Space2 Eye Lens: State of the Bog Dark Peak Condition Survey, Space2 Eye Lens: State of the Bog (MMU) Mountain Hare Habitat Survey Dark Peak, Uplands Inventory, West Pennines Designation NVC Survey, Wetland Inventories, WorldClim - Global Climate DataFull metadata can be viewed on data.gov.uk.

This resource contains boundary, site and feature information for Special Protection Areas (SPAs) with marine components in UK waters. The files were last updated in November 2019.

Special Protection Areas (SPAs) with "marine components" protect bird species that are dependent on the marine environment for all or part of their lifecycle, where these species are found in association with intertidal or subtidal habitats within the site. More information on SPAs with marine components can be found on our website at: http://archive.jncc.gov.uk/page-4559. SPAs with marine components are a subset of the UK's full suite of SPAs - data for all UK SPAs can be download from our website at: https://jncc.gov.uk/our-work/uk-protected-area-datasets-for-download.

JNCC collate boundary and attribute data on Special Protection Areas (SPAs) on behalf of the Country Nature Conservation Bodies (CNCBs) to create a dataset of inshore and offshore sites across the UK. Please note, the CNCBs periodically update SPA boundaries (e.g. when natural features of the coastline shift position) and these subtle changes may not be captured in JNCCâ s datasets. For the latest inshore SPA boundaries, please visit the relevant CNCB website (https://jncc.gov.uk/our-work/uk-marine-protected-area-datasets-for-download/#cncbinfo).

A shapefile and spreadsheet are available for download. The shapefile contains site boundaries and associated site attribute information, including: site code, site name, site status, country and CP2 region. The spreadsheet provides the same site attribute data as well as information on the sites' protected features (including species name and population type).

For further information please see the shapefile metadata and spreadsheet read-me (within the files).

The division into counties based on the model used in the English system, was introduced gradually by English settlers from the late 12th century onwards. These land divisions were formed following the Norman invasion of Ireland in imitation of the counties then in use as units of local government in the Kingdom of England. The older term "shire" was historically equivalent to "county". The principal function of the county was to impose royal control in the areas of taxation, security and the administration of justice at local level. Following a survey under the 1825 Boundary Survey Act, an extensive series of maps of Ireland was created by the Irish division of the Tailte Éireann for taxation purposes. These maps both documented and standardised the boundaries of the thirty two counties of Ireland.Coordinate Reference System: Irish Transverse Mercator.This dataset is provided by Tailte Éireann

This dataset contains gridded human population with a spatial resolution of 1 km x 1 km for the UK based on Census 2011 and Land Cover Map 2015 input data. Data on population distribution for the United Kingdom is available from statistical offices in England, Wales, Northern Ireland and Scotland and provided to the public e.g. via the Office for National Statistics (ONS). Population data is typically provided in tabular form or, based on a range of different geographical units, in file types for geographical information systems (GIS), for instance as ESRI Shapefiles. The geographical units reflect administrative boundaries at different levels of detail, from Devolved Administration to Output Areas (OA), wards or intermediate geographies. While the presentation of data on the level of these geographical units is useful for statistical purposes, accounting for spatial variability for instance of environmental determinants of public health requires a more spatially homogeneous population distribution. For this purpose, the dataset presented here combines 2011 UK Census population data on Output Area level with Land Cover Map 2015 land-use classes 'urban' and 'suburban' to create a consistent and comprehensive gridded population data product at 1 km x 1 km spatial resolution. The mapping product is based on British National Grid (OSGB36 datum).

This dataset is refreshed on a weekly basis from the datasets the team works on daily.Last update date: 11 July 2025.National Highways Operational Highway Boundary (RedLine) maps out the land belonging to the highway for the whole Strategic Road Network (SRN). It comprises two layers; one being the an outline and another showing the registration status / category of land of land that makes up the boundary. Due to the process involved in creating junctions with local highway authority (LHA) roads, land in this dataset may represent LHA highway (owned by National Highways but the responsibility of the LHA to maintain). Surplus land or land held for future projects does not form part of this dataset.The highway boundary is derived from:Ordnance Survey Mastermap Topography,HM Land Registry National Polygon Service (National Highway titles only), andplots researched and digitised during the course of the RedLine Boundary Project.The boundary is split into categories describing the decisions made for particular plots of land. These categories are as follows:Auto-RedLine category is for plots created from an automated process using Ordnance Survey MasterMap Topography as a base. Land is not registered under National Highways' name. For example, but not limited to, unregistered ‘ancient’ highway vested in Highways England, or bridge carrying highways over a rail line.NH Title within RedLine category is for plots created from Land Registry Cadastral parcels whose proprietor is National Highways or a predecessor. Land in this category is within the highway boundary (audited) or meets a certain threshold by the algorithm.NH Title outside RedLine category is for plots created in the same way as above but these areas are thought to be outside the highway boundary. Where the Confidence is Low, land in this category is yet to be audited. Where the Confidence is High, land in this category has been reviewed and audited as outside our operational boundary.National Highways (Technician) Data category is for plots created by National Highways, digitised land parcels relating to highway land that is not registered, not yet registered or un-registerable.Road in Tunnel category, created using tunnel outlines from Ordnance Survey MasterMap Topography data. These represent tunnels on Highways England’s network. Land is not registered under National Highways' name, but land above the tunnel may be in National Highways’ title. Please refer to the definitive land ownership records held at HM Land Registry.The process attribute details how the decision was made for the particular plot of land. These are as follows:Automated category denotes data produced by an automated process. These areas are yet to be audited by the company.Audited category denotes data that has been audited by the company.Technician Data (Awaiting Audit) category denotes data that was created by National Highways but is yet to be audited and confirmed as final.The confidence attribute details how confident you can be in the decision. This attribute is derived from both the decisions made during the building of the underlying automated dataset as well as whether the section has been researched and/or audited by National Highways staff. These are as follows:High category denotes land that has a high probability of being within the RedLine boundary. These areas typically are audited or are features that are close to or on the highway.Moderate category denotes land that is likely to be within the highway boundary but is subject to change once the area has been audited.Low category denotes land that is less likely to be within the highway boundary. These plots typically represent Highways England registered land that the automated process has marked as outside the highway boundary.Please note that this dataset is indicative only. For queries about this dataset please contact the GIS and Research Team.

Output Area is the lowest geographical level at which census estimates are provided. Output Areas were introduced in Scotland at the 1981 Census and in all the countries of the UK at the 2001 Census.

The Output Areas and Small Areas list contains 232,296 areas of the following constituent geographies:

• 171,372 Output Areas in England
• 46,351 Output Areas in Scotland
• 4,537 Small Areas in Northern Ireland
• 10,036 Output Areas in Wales

Please visit ONS Beginner's Guide to UK Geography for more info.

The boundaries are available as either extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands) or

clipped to the coastline (Mean High Water mark).

This is the ONS Postcode Directory (ONSPD) for the United Kingdom as at February 2023 in Comma Separated Variable (CSV) and ASCII text (TXT) formats. This file contains the multi CSVs so that postcode areas can be opened in MS Excel. To download the zip file click the Download button. The ONSPD relates both current and terminated postcodes in the United Kingdom to a range of current statutory administrative, electoral, health and other area geographies. It also links postcodes to pre-2002 health areas, 1991 Census enumeration districts for England and Wales, 2001 Census Output Areas (OA) and Super Output Areas (SOA) for England and Wales, 2001 Census OAs and SOAs for Northern Ireland and 2001 Census OAs and Data Zones (DZ) for Scotland. It now contains 2021 Census OAs and SOAs for England and Wales. It helps support the production of area based statistics from postcoded data. The ONSPD is produced by ONS Geography, who provide geographic support to the Office for National Statistics (ONS) and geographic services used by other organisations. The ONSPD is issued quarterly. (File size - 234 MB)NOTE: The 2022 ONSPDs included an incorrect update of the ITL field with two LA changes in Northamptonshire. This error has been corrected from the February 2023 ONSPD.NOTE: There was an issue with the originally published file where some change orders yet to be included in OS Boundary-LineÔ (including The Cumbria (Structural Changes) Order 2022, The North Yorkshire (Structural Changes) Order 2022 and The Somerset (Structural Changes) Order 2022) were mistakenly implemented for terminated postcodes. Version 2 corrects this, so that ward codes E05014171–E05014393 are not yet included. Please note that this product contains Royal Mail, Gridlink, LPS (Northern Ireland), Ordnance Survey and ONS Intellectual Property Rights.

Super Output Areas are a geographic hierarchy designed to improve the reporting of small-area statistics.

The Lower Super Output Areas and Data Zones list contains 42,619 areas of the following constituent geographies:

• 32,844 Lower Super Output Areas in England
• 890 Super Output Areas in Northern Ireland
• 6,976 Data Zones in Scotland
• 1,909 Lower Super Output Areas in Wales

Please visit ONS Beginner's Guide to UK Geography for more info.

The boundaries are available as either extent of the realm (usually this is the Mean Low Water mark but in some cases boundaries extend beyond this to include off shore islands) or

clipped to the coastline (Mean High Water mark).