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. 
>_<