In London I don't really bother with Forbidden Planet much anymore, they tend to cost the earth and hardly ever have early volumes, at least that's in my experience. I like to look around because they have so much there, I use them as the benchmark, but I rarely buy from them. Although they do have a healthy selection of artbooks, which I don't normally see elsewhere.
Gosh is ace, often I find them cheaper than FP too, plus they normally have the early volumes of series in stock.
Orbital Manga is another freakin' ace place, I only recently found it, but they're more like FP used to be. Some of their prices are nice and cheap but others can be more expensive than FP, plus they stock Project Arms which means I can catch up. I agree with the whole buying everything ethos, I see all kinds of cool little trinkets and just can't resist.
There's a Japanese bookshop in Soho, which I went to a few years back and I love it, but sadly everything is in Japanese, I think it's still there, not sure what prices are like. The window has some Manga/Anime posters in it, I bought some random games mags from there once ... just to look at the pretty pictures