Well, I was getting desperate so I installed the French version. Still nothing happened, so I used the update tool again (thinking it would revert to the standard UK one); in the end the problem turned out to be a hardware fault. Just got the phone back, and the keyboard is now French (q and a are mixed up, as are z and w), while the text is in English. I'm planning to try and update to 2.3 custom ROM anyway, so I'm hoping this will take care of the keyboard also. Will keep you updated, but just to let you know to be a bit careful.
Does no one have a UK stock rom that they can post?