Best I`ve used so far are Play247.com they seem to be based in Jersey so discs take a couple of days to arrive, but are reasonably priced and include postage!
By the way, anyone noticed Blackstar seem to be getting quite aggressive with their Pre-Order discounts for R2 titles? I`ve ordered the 2 series of Cold Feet coming out soon, and they`ve got them for £15.99 inc. postage (as opposed to Amazon`s £23.99 PLUS postage- "You save £1.00"!!).