Near the books area, there's always a place where
they put cels and other Japanese books. Might
take you a while to find the place, it's not very
obvious. They just put all the cels in a
cardboard box for you to flick through. Don't
expect to find popular series like Kenshin or
Sailormoon, although I did see some GTO cels.
Also, I was at Covent Garden (Neal Street) last
weekend and saw one of the street vendors selling
cels for a tenner each. He just put a blanket on
the floor and put the cels on it. It was scary.
Particularly because it was going to rain soon.