The Blue Cross are also excellent for rehoming Cats and Dogs. I'd use them for my next mog(s), but none of the Blue Cross centres seems to rehome cats in London (or at least not the Croydon part of it anyway).
I got my old cat from the Blue Cross in Burford, and the amount of info they had on each animal was impressive. They detailed the history where they had it, and what sort of environment the animal needed etc. Do a google for the Blue Cross website, and see if they have a centre near you.
Some of the CPL groups appear to be similar, although they do seem to be more dependent on lots of volunteers, and hence aren't as centralised as organisations like the Blue Cross, and Battersea.
Once I've got the house reasonably sorted out, I'll be off to find some little furry buggers, although I like pre-owned adult cats, they're probably better off to be left by themselves than kittens would be.