I would recommend 7dayshop.com. Usually unbeatable for all things Camera. Ships from Guernsey involving some sort of tax loophole.
If you only want a memory card you could try ebuyer.co.uk. Not quite as cheap but you can often pick up items with free shipping, which usually makes them cheaper than ebay!
Either way, dont, dont, dont buy them from UK high street chains or you will be fleeced something chronic, even so called tax free shops at the airports.