I go to piccadilly cicus all the time and i have never seen any jpop cd's in any shops except for one time about a year ago, lol. the virgin opposite the trocadero had it about a year ago, it was l'arc~en~ciel's AWAKE album and DO AS INFINITY's NEW WORLD album was there. They were both £30 and i just couldnt justify it. They are both awesome albums but i can get it cheaper online like edgefusion said. i go to the Japan centre like every other weekend and i have never heard of them selling cd's though. i know china town like the back of my hand and you will only find chinese cd's there, and even now some of the shops got closed down there is less choice for even that. So yeah, sorry there is hardly any (if at all) jpop stuff in london, but if you want anime or manga there are some places.
lol just realised i missed you by a day, sorry i am tired