Suction is quite right - whilst you cannot train in Denham in the overhead or nearby to it (but where can you in the souther-east anyway?) the transit north is in itself a learning experience. Denham has a tighter and more regulated (due to the Heathrow zone) approach and circuit than most airfields where you might train. THis can only be good for your circuit skills, radio, nav etc.
I converted my NZ PPL to UK at TPC at Denham and later did a multi engine at Cabair and I found both to be excellent - Denham's proximity to London saved time driving and so made flying more accessible - also a consideration for many people.
My 2 cents worth.