In a discussion on the Ephotozine forums about Phil Smith being stopped by Ipswich police and requested to delete his images, one site member, Dave (going by the pseudonym of Mad-Dog's) detailed a card he carries summarising the law concerning photography in the UK.
I have created my own version of the card and you can download it as a jpg from here.
Printed at 300dpi this should produce a credit card sized document that you can easily carry with you and may help you challenge any over-zealous police officers you may meet.
Public Photography and the Law
Jobsworths and not-so-jobsworths