Andrews & Arnold do colocation as well. If you don't mind using a VIA 1GHz then they've got a 50GBP a month thing going. You have to buy the server up front for 500GBP however but it does mean you keep the thing if you ever decide to stop. It's got 512MB RAM and a 180GB drive and you're allowed about 300GB a month transfer.

As usual, I've nothing to do with A&A apart from I use their services.