I love comics - and manga, graphic novels, etc. I have to admit that if it weren't for my brother and some friends I probably wouldn't know anything about that, but since I was a kid me and my brother would always read Dragon Ball and Marvel comics, and later on I started reading Naruto and graphic novels from great authors like Craig Thompson or Nate Powell.
All of this and more can be found at Forbidden Planet, a big store located next to Covent Garden. It's one of the most large and best-known science-fiction and fantasy stores in the world, and it has several stores in the United Kingdom, the biggest one being the one in London.
And indeed they have LOTS of things. For starters, as you go in there are stands with awesome candy boxes. I love the Nintendo inspired ones!
On the ground floor there are tons of action figures from Nintendo, World Of Warcraft, Animal Crossing, Nightmare before Christmas, Marvel, Transformers, and much more. There are also vinyl toys:
And on the basement they have a big selection of comics, manga, and graphic novels. I really don't know that much about it, so maybe if you're looking for something really exclusive you won't find it, but I think they do have a lot!
And that's pretty much it. I really recommend it!