The OS Open USRN product uses the British National Grid (BNG) coordinate reference system (CRS).
BNG uses the OSGB36 (EPSG 27700) 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 specify a vertical (height) reference system.
Please view our 'Guide to Coordinate Systems in Great Britain' for more information on CRSs.