There is a place called Colindale and large 
shopping Mall type area called Oriental City, 
this has several japanese related store aswell 
as the largest Japanese Bookshop in Europe.

As for anime cels, there is nothing at all 
really... 

Forbidden Prices is on New Oxford Street, the 
nearest tube is tottenham court road.

The Japan Center is at Piccadily and China Town 
is about 5 mins walk from here. But like wat was 
said in an earlier post, there are alot of 
copies and stuff that is not official.

There is a small japanese bookshop on kensington 
street about 10 mins walk from China Town.

And that's really it,

There is the London Anime club that also happens 
on the first sunuday of every month, that's 
where you'll pick up the best anime stuff, 
around 5/6 dealers.

That's about it!!