We've used the Oakley Hotel for Cats for years - it's lovely. It's on the far side of Bromley, about 20 - 30 minutes drive from Forest Hill. It's quite large, but they have lots of different sections and if your cats don't want to mingle, they can be kept separate. The individual runs have plenty of room, and there are also large communal areas full of old chairs and cupboards where the cats love to find snug corners to sleep.

A quick google search should bring up their website for you.