Phone arround and try and find an indipendant guy, I've used two guys fairly local to me, Dave (D&G Rider Training) hes in bromley (a bit far from you!), and Steve - freedom rider training (a different one to spidermans though.
http://www.freedomridertraining.com/ hes in swanley or essex.
I did a DAS at a fairly big place, in swanley and there attitude stank. It was a big con, just had us rideing arround there yard, swapping bikes over, "I've taught more people to ride than you have had hot dinners". No shock i failed my das with them people, and didnt go back for my free retraining. It was really like a pass your test boot camp! Really try to avoid these!!
After i failed my first test, I found Dave (D&G) did a two day course with him, failed because a car pulled out on me, booked another test and he gave me a days free training, as he had a spare day! Needless to say i passed that test no worries.
I've just done some advanced training with Steve (freedom), and feel a lot more confident now. He does DAS as well.
These guys work for there own company, and really have a passion for what they do and care about there company image! So you may find you get more value for money at one!
Dave even holds a BBQ in the summer for all his Ex students!
Dan