Joe,
Not sure about the London area but I live on the West of London and drove to Norwich to do mine. American Flight Training does both the theory and flying training. The flight test is either conducted in the US or there is a visiting examiner they can arrange for you I believe.
I just completed an Instrument Proficiency check with AFT this weekend. Rates are reasonable and a good choice of aircraft - including a nicely kitted out Aztec if you are feeling flush!
Julian.