Hmm, I've not seen it for ages but you might have luck checking out Games Focus. They have a great selection of new and used games on all formats, including some retro and import stuff.
To get there, come out of Goodge Street tube station, turn right and then take the second street on the right - there's a 'Samurai' sushi bar on the corner of the road, so turn right when you see that. Game Focus is about 8 stores down, on your left.