Ordnance Survey National Grid

The Ordnance Survey National Grid reference system (OSGB), also known as British National Grid (BNG), is a system of geographic grid references, distinct from latitude and longitude, whereby any location in Great Britain can be described in terms of its distance from the origin (0, 0), which lies to the west of the Isles of Scilly. Devised by the Ordnance Survey (OS), the national grid reference system is heavily used in OS survey data and in maps based on those surveys, whether published by the Ordnance Survey or by commercial map producers.

Source: Wikipedia — Ordnance Survey National Grid (CC BY-SA 4.0)