Forbidden Planet London is a great store, probably the best comic book/movie/sci-fi/pop culture store in London. The entire ground floor is full of merchandise, t-shirts, toys, statues, collectibles etc. from movies, video games, TV shows, comic books, anime, and more. The entire basement is then full of comic books, graphic novels, magazines, 'making of' and 'art of' books, novels, fantasy novels, and a whole lot more.
Gasdoc is right in saying that it is overpriced. It is extremely expensive in there. But in terms of physical stock and stock variety, there is no better place. I always go there to check out products, and then normally order online. Graphic Novels alone can be 20-30% cheaper on Amazon.
You should also check out The Cinema Store on Upper St Martin's Lane and Orbital Comics on Great Newport Street.