try - http://www.photobox.co.uk/ you get 30 free - I use them and they're great...

http://www.truprint.co.uk/registration - haven't used them myself, but you get 20 free...

http://www.jessopspicturehouse.com/google_print.html - again 20 free...

I just did a google on - photo prints uk

there will no doubt be loads more!