Online photo printers SnapMad are doing free delivery until 28th February, with no minimum order value! Prices are comparable with Snapfish:
6x4 = 10p
7x5 = 17p
I tried them out for the first time this week: 3 photos delivered for 37p! Delivery took 3 days. The package wasn't as slick as Snapfish but the quality of the photos was fine. Postage alone cost them 37p!