I managed to get a copy of Team Fortress 2 from PC World for £3.50. Toys R Us are selling a lot of games for £4.99. One of these is Team Fortress 2. Go into a PC World, pick up a copy of one of the games, and then ask to price match it with Toys R Us. When you price match, you get the price matched plus 10%. Normally, it costs £19.99 in PC World - so with the price match you save £16.50 on the PC World price or £1.50 on the Toys R Us price! Other games you can get at this price include Portal and Stranglehold.
Just so you know, this was tested on the Thurrock/Lakeside PC World and Toys R Us stores.