I heard from someone who should be a reliable source that you need to allow up to 300m for error. So mind how you go round TMA/CTZ's etc.
Also read your Notams carefully - the "powers that be" carry out GPS jamming trials in some far flung parts of the U.K. Whether or not they are succeeding would be interesting to know - presumably they can do this to stuff the enemy in the event of blighty being attacked :o