I wondered about that too as I wrote my response, but I haven't traveled outside the U.S. since I've had my GPS (should have brought it to Taiwan once, but didn't). Thanks for the correction.

Here is a very good link to information on the British coordinate system:

http://www.gps.gov.uk/guide1.asp

It appears to work similarly to the UTM grid system.

Is there a "standard" source of UK maps - something similar to the USGS in the US?