I manually went through half a dozen dictionary files - all of them were terrible, full of things like "connexion", and even words that haven't been used since Chaucer's time. The online dictionaries aren't much use either, all the American ones are wrong, and the OED is far too out of date to be of any use (Britain is absorbing a lot of American words and spellings at the moment)
Edit: And let this be a lesson to anyone else trying to localize a website from en-US to en-GB: Every reference you have to hand is wrong. If you don't get an actual British English speaker to look over it, we will laugh at you.
Edit2: Dug up some of my notes from an en-GB <-> en-US dictionary file we attempted to use:
We don't use "gaol" or "gramme" anymore
"Ton" and "tonne" are different measurements, not translations of each other
"Practise" and "practice" are not interchangeable
"Check" cannot be globally replaced with "cheque"
"Terrorise" is not and has never been a word