I think I've passed on these recommendations before.. Not in London but quick via post -I've been using Peak Imaging for about 8 years and they are really good (to me, anyway ho ho), but for process & scan to CD when I'm in a hurry I use Genie Imaging in South London and DS Colour Labs (via post again)- both pretty cheap and good quality.
Nowehre else in London is as cheap as Genie - that i could find anyway..