It's £3 (or £1.50 if you're a student) for E6 5x4 at the Vault in Brighton and they've always done an excellent job on all of the 35mm and 120 films I've taken in there. I'm not sure whether you can post things there though, but it's worth an email to ask.
Wow £1.50 is very cheap, do you have to be studying photography to be counted as a student?
I'd get it processed by someone you can rely on, Peak Imaging will do it for £2.50 and are by all accounts excellent.