3MF/Dave,
Mike Cross probably has it right - if you're using the in-built mapping it will be "slightly" inaccurate for ground features, and will probably be using WGS 84 internally.
If you want to check out the accuracy of the GPS (and not the in-built mapping!), use a real map (1:50000 Landranger or 1:25000 better), set the format to British Grid and datum to OS GB. Plot your position on the map, see how close to real life it is, then alter the datum and see how much it differs!
If using maps for flight planning/VFR navigation aided by GPS, then correct datum is important - real maps should have the datum used in the legend, electronic ones should specify somewhere!