This is an online book store that I know quite a few people have used in the past. Their sale is not on, and they claim to have books from £1 - possibly worth checking out and seeing what offers there are. There is automatic free shipping on orders of £25 or more, but there are also free shipping codes in our voucher section if you want to try them out.
This store has a great children's selection as well. Thanks to NorthernGirl for the heads up.