Advance Wars: Dark Conflict for the Nintendo DS only £4.99 at Comet!
Can order online (but you'll have £5.95 postage to pay) or reserve online and collect from store for FREE! (Edit: Can no longer select Collect from Store on the website, but if you go into your local store, and ask them to order product number 438367, it can take 3-5 working days. Thanks to Saintscouple for this info!)
Absolute bargain!! (CEX currently giving £16 store credit for this!)
Blockbusters are offering £14 trade for this game, and £8.40 cash - plus there is an offer on where you get 50% extra on cash trade-ins at the moment, so you'll get £12.60 cash back! Bargain!
Also, go through Quidco for 4.5%, (now includes click and collect purchases).