Japan Center is very close to Piccadilly Circus, I can't describe which side street it is, but when you're standing on Piccadilly Circus, just look around and in one of the streets you should see Japan Center, just a couple of yards from the Circus.
It's it's your left-hand side (from the direction of Piccadilly Circus) and across the street is a Starbucks.
I'm not familiar in London sorry
but I do remember that.
I went in December 2008, and they had 3 magazines with Ayu on the cover!! So cool! I bought Scawaii