WGS84 (decimal) — Two numbers that fit latitude/longitude ranges (e.g. 51, 2 or 51.5074, -0.1278) are read as degrees, with ordering heuristics for lat/lon vs lon/lat.
BNG / OSGB36 (metres) — Two large non-negative values in plausible easting & northing (metres, with a minimum scale so tiny pairs are not BNG) are treated as British National Grid, e.g. 530276, 180641.
OS grid reference — Letter pairs plus digits (e.g. TQ 30080 80508) are parsed as a grid ref before plain two-number lines, so the digits are not mistaken for raw metre coordinates alone.
WKT / EWKT — Use POINT(…) with optional SRID=27700; (or 7405) for explicit BNG, or geographic SRIDs such as 4326 for lon/lat.