Jacqueline van Roosmalen (
www.vanrosetherapy.com) in Dulwich is brilliant, but very booked up so you'll have limited choice of time slots. However, she is a child/adolescent psychotherapist not a psychologist (so not a CBT approach).
I do think it's important to understand the underlying issues before going to cognitive techniques so would give her a call.
Redfern Psychology in Streatham was used by a couple of people I know and they were pleased - they have clinical psychologists/therapists, etc
The Child and Family Practice has a multi-disciplinary team so might be a a good first port of call to see who is the most appropriate specialist to start with (
http://www.tcfp.org.uk/)
Well done for addressing the issue - so many friends/acquaintances hope "it's a phase" and things only get worse (different issues, not just anger management) whereas sometimes in 10 sessions you get a transformation.