If your prepared to travel a little east of London into Kent then I'd recommend The Tiger Club. They operate from Headcorn Aerodrome, IMO one of the UK's most interesting aerodromes. They have a couple of different aerobatic types but their CAP 10b upgraded with a carbon wing seems to be the choice. The hire rates are reasonable and there are no instructor fee's! See
www.tigerclub.co.uk .