For those who don't know: You need OSGB34 for the U.K. Stands for Ordinance Survey Great Britain 1934. It's a a better fit than WGS84 (World Geographic Survey 1984). That is due to the Earth being an oblate spheroid. With local variations in the Earth's curvature.

It's not a major issue if you are using OSGB BSG (British Standard Grid). It is an issue if you are using latitude and longitude or UTM (Universal Transverse Mercantor). Although I have to point out that modern GPS units have a level of accuracy that far exceeds that of the OSGB Maps (5.0 meters for 1/25000 or 10.0 meters for 1/50000).