I just want to put in another good word for dscolourlabs.co.uk. I often get enlargements (usually 18x12), and from photobox this costs £5.99 a go. I tried dscolourlabs and found they were not only cheaper £0.99 (!), but also, they package them so well!
Photobox would roll the prints, which is fine if you are going to frame them, but can be annoying nonetheless. DSColourlabs post them flat, between several sheets of corrugated plastic and them place them in a cardboard box. The colour's themselves appear a little more vibrant, but this seems to vary a little bit. Overall, I'm very pleased and will be saving myself £5 on each print from here on in.