I used to do it a while back and I recommend it highly- stopped doing it for reasons of money and time. It gets you very fit, it's challenging, it makes you feel a bit harder which is good in London, and the place I went had a really nice vibe where it wasn't aggressive or anything. At the time, I had some problems with a knee injury, and the kind of squats and stuff we did really helped stengthen my knee. There were a lot of girls there, too, which makes for a nice atmosphere. But it was still damn challenging, I count getting a red belt as one of the hardest things I've done, mentally and physically.
Anyway, the place I went is called Springhealth kickboxing (
http://www.springhealth.co.uk/) on York Road, off Camden Road. Lessons were about 8 quid for group lessons, which seemed worth it. Nice people.
Any more questions you've got, let me know! I know the people there fairly well.