I have to say I disagree. It's true they have fewer miniatures, but in terms of RPG material, I think they have more. It's just not displayed as well.
The 3 shops listed so far: Orc's Nest, Playin' Games and Leisure Games are the main RPG stores in London. Orc's Nest and Playin' Games are a walking distance away from each other, as is Forbidden Planet. So you (or your GF) can do all three of them at once.
There are a few other places, but none worth making a special trip for. There's And So it Begins in Ealing shopping centre, opposite the Games Workshop. They have some RPG material. There's also They Walk Among Us in Richmond. I haven't been there, but according to fellow EN Worlder Tallarn, they have some RPG's too. As I said, probably not worth going out of your way for either of these.