My first deal, so be gentle :)
Popped into PC World to exchange something, and noticed they had lots of their games reduced, as per previous threads, and as mentioned by some, the BOGOF has started already. However, I noticed they had Halo 3 Limited Edition (the one in the metal tin case, but not with the helmet) for £9.97. Then, noticed they were BOGOF too.
So I walked out with 2 copies of the game for £9.97, CEX will take £14 trade in for these each, or £10 cash, and Gamestation will beat both those prices by £1, so you've effectively got (best case scenario) £30 worth of spend at Gamestation for £9.97 ;)