I've used Photobox in the past and been very happy with the results. I had my portfolio for the Royal Photographic Society printed by them, and the quality was excellent.
However, that was back in July, and I've read one or two posts elsewhere from people who feel that the quality has gone downhill recently, so I don't know. You could try a couple of test prints from Photobox (they're pretty cheap) and see what you think.
There's one local to me (Bristol) -
Photographique Bristol - Quality Photographic Printing and Gallery - slightly higher prices than Photobox, but although I haven't used them myself yet, I do know a couple of people who've had nothing but good things to say about not just the quality of the prints, but the quick turnaround, and excellent service too.
Russ.