Notes on precision. Latitude/longitude here are WGS84 (what satnav gives you). The Ordnance Survey reference is produced by converting to the Airy 1830 ellipsoid with the standard OSTN-approximating 7-parameter Helmert transform: expect agreement with the printed grid to within a few metres, which is fine for walking, not for surveying. A six-figure grid reference locates a 100 m square, eight figures a 10 m square. Geohash and Plus Code shorten by dropping precision, so a 6-character geohash is a ~1.2 km box while an 11-character Plus Code is roughly 3 m.