Where to buy J-Pop in London?

[xLuna&1LOVE]

I don't think you will find J-Pop so easily in western countries. Ayumi Hamasaki isn't very popular in our environment a.k.a. Europe.
You can buy those CD's, DVD'S and fanstuff (like Ayupan) from:

ebay, but ok... maybe isn't that good quality.
yesasia.com This is where I buy Jpopalbums. You have to find it out how it works, but we will help you. It's really easy.
amazon.com Warning! This site isn't always reliable.
cdjapan.com (or whatever) I never look on this site, but a lot of members here use this site.

Well good luck, and I hope a I helped you a little bit.