I really like the results at Nation's Photo lab and do not be fooled, they are not all the same. I feel like they have the best color matching that I have found. The shipping is a bit high, so I try to do one big order instead of a lot of little ones, but I feel the print prices are quite reasonable. http://www.nationsphotolab.com/allprints.aspx
I use photobox, they always have offers and at the moment have 40% off. Their service is first class. I ordered 12 x A3 calendars with my own photos last Thursday evening, they were delivered on Saturday!! http://www.photobox.co.uk/
I see you are in the UK. I always check out this site for value for money - but let others advise about quality. I'd be interested in that too.
http://www.nationsphotolab.com/allprints.aspx