Just been in PC World Manchester, near Old Trafford. They have a pretty large selection of older game on clearance, buy one get one free. Most around a tenner, some for more, and a few really old/shovelware games for £1-5
They have DS, PSP, PS2, Xbox 360 and one or two PS3 games available. I was nearly tempted by Yakuza for £10, but couldn't find another PS2 game I wanted (you can't mix and match formats).