OS Open UPRN has two coordinate reference systems (CRSs) present within the data:
British National Grid (BNG)
European Terrestrial Reference System 89 (ETRS89)
BNG 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 specify a vertical (height) reference system.
ETRS89 is the EU recommended frame of reference for European data and is represented as Latitude and Longitude values. ETRS89 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.