Hi
Five5 - As you live in Bucks. I would recommend
Brtish Airways Flying Club - very good fleet, well maintained, and very high standards.
Re- the airspace in SE UK / NW London area - I'd say its a definite advantage for you to learn in such an environment in the long run. You will become accustomed to different types of ATC environments (TWR, AFIS, AG, Class D, LARS) early on and not be phased - I have heard e.g. that some who learn in the open skies of the US may be daunted on return to this environment, so I reckon best of starting here.
Hope this helps,