Alex, I can highly highly recommend a woman called Colette Kase who holds puppy classes near Baker St tube. Would that be close enough?
She takes puppies from after the 1st injection (7 wks) and I think this early socialisation is irreplaceable. All other pups there are at a similar level of immunity (1st jabs only).
She is a member of the APBC and uses only positive methods and her course is roll-on, roll-off - so you can go any week, and you can keep going until your puppy reaches the maximum age of 20 wks - so that's 13 wks of training, if you go from 7-20 wks.
The cost is £100, but this is a flat fee and for 13 wks of training isn't bad. It's also a central London class, so is slightly more expensive for that reason - hall hire at Baker St can't be cheap.
Her website is here:
http://www.dogclub.co.uk/ckase/pups.shtml