As much as I hate to promote this game further, here it is! Wrath of the Lich King
Went to my local WHSmith in Plymouth today and discovered this deal. Seems to be instore only, can't find it on the website. The next lowest price around is generally £15 for this game, so a half price bargain! Ideal if you don't have this expansion yet and are planning to buy the next instalment of Warcraft - Cataclysm.
Can't guarantee this is nationwide, but still worth a look if you're near Smith's.