Costco def cheapest I can find for a brick of 12 at £17.89 .... unfortunately they only come mixed - 6 red and 6 blue.
Cards4magic do a brick of 12 blue for £21.00. Free postage on orders over £50, but I can't find how much postage is for orders below £50 .... I'm guessing it'll be a few quid.
I've found a seller on Ebay, selling single decks, but with a discount for buying more. 12 decks for £22.99 + free postage.
I've found some slightly cheaper on Ebay; but the fact that they're coming from China, makes me question whether they're the real McCoy or not
I'll put the Ebay link here, but I know it'll be out of date soon; but it's here if anyone can make use of it:
http://www.ebay.co.uk/itm/291800675901? ... EBIDX%3AIT