If we were just using Global Positioning system (GPS), that would be the end of the story. However, we are often trying to relate our position to features on the map, which causes additional problems and errors, because GPS and Ordnance Survey maps use different models for the earth and coordinate systems.
This is especially the case with height because the model that relates the global coordinate system that GPS uses, WGS84, to map height above sea level is very poor. WGS84 is a modern coordinate system, but maps in Britain are based on the National Grid, which uses an older system known as OSGB36.
Although it was surveyed very well for its day, OSGB36 has distortions in it, which have to be taken into account when comparing with GPS-derived coordinates. GPS receivers can correct for the large main offset between the two systems, and it is vitally important to set the receiver's datum (the definition of the coordinate system used) to OSGB36 so that it does so.
Please refer to your receiver's manual for information on how to get your receiver to work with Ordnance Survey maps. Smaller level distortions are still present which could add up to 5m errors when using GPS with Ordnance Survey maps. Most receivers can cope with a variety of coordinate systems for different countries.
To read more detailed information on datum transformations, see From one coordinate system to another: geodetic transformations in A guide to coordinate systems in Great Britain (PDF).
On top of the nationwide errors in OSGB36, individual features on the map may only have been surveyed to a local accuracy of 7m (for 1:25,000 scale maps) and some features, such as boulders, may only be shown schematically. Of course, this last problem affects anyone trying to position themselves on the map, not just GPS users.