The coordinate reference system used by OS MasterMap Highways Network is the British National Grid. The British National Grid (BNG) spatial reference system uses the OSGB36 geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using easting and northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not include a vertical (height) reference system.
In the Geography Markup Language (GML) data, this is represented by reference to its entry in the EPSG registry, as http://www.opengis.net/def/crs/EPSG/0/27700.