World of Warcraft: Burning Crusade expansion pack, £15.99 delivered at powerplaydirect.com. I'd suggest being fast since 8 million people own the original and this is a pretty cheap preorder. Note that you still have to pay monthly if you didn't know that already...
This is the first time I've done this, and I searched the forums for Burning Crusade beforehand and nothing came up, but tell me if I've done anythign wrong please :)
EDIT: Yes, you do need the original to play this - although if you have the original I'd suggest buying this as well, since it's cheap and the monthly payments are really where it costs you. The original WoW can be found on here for about £10, if anyone is thinking of buying.