Try this link:

htt
p://www.forbidden-
planet.org/Anime/Shopping/shopping.london.txt

The stores I've been to (there were no cel shops 
when I was there, 5 years ago) are:

Books Nippon (St. Paul's Churchyard)
- A nice small manga store with a great selection 
of artbooks. This is where I bought most of my 
shoujo manga.

Forbidden Planet
- More of a sci-fi/fantasy store, but they carry 
more and more manga and anime stuff all the time. 
This is a big comic mecca in London.

Japan Centre (Picadilly)
http://www.japancentre.com/
This has a full-size Japanese bookstore (it 
wasn't a Kinokuniya, but it's the same size as a 
Kinokuniya). Also it has one of the best 
Okonomiyakai restaurants outside of Japan (I hope 
it's still there!) You can also find the typical 
Japanese stationery shops, so you can probably 
find cel books here. This -is- the Japanese hub 
in London, but it's kind of tricky to get to. 
Worth the trip though, I usually went down 2 
times a month when I lived in the UK. If you are 
going to find cels anywhere, it's probably here.

I also know of several excellent shops in 
Sheffield and Leeds if you are going that far 
north. Contact me offline if you would like to 
know more http://www.anime-online.com/email.html

take care!
misa