I am currently doing my PPL at The Pilot Centre and am very happy there.
Very friendly bunch, any instructor is happy to chat with you about questions you may have if your instructor is flying (useful when studying for the exams).
Before I started taking lessons, I had some thoughts on whether I should learn on a low-wing PA28 or high-wing Cessna. Denham has both although the PA28's are more expensive. In the end, it makes no difference from my perspective. For the purpose of learning to fly, a C152 is fine (and forgiving although somewhat cosy!), you can always upgrade/change later.
An important factor if you live in Central London is how long it will take you to get to Denham or Stanmore. I live near Paddington so a drive up the A40 one way is about 25 minutes. Bearing in mind you will have up to 60 minutes brief before flying for about 60 minutes, you are spending 2 hours at TPC plus 1 hour commute, it knocks out 3 hours of your weekend easily (if all runs smoothly). A pain if you go there and your lesson then gets weathered out.
If you are using public transport, consider what works best. Whilst you can always study during the journey to and from the aerodrome, it should be conveniently accessible. I have 2 children so weekend time is premium (and negotiation with wife). It helps that my office is now near Northolt so I can occasionally have an early morning or late afternoon lesson...
You could always do a trial lesson at each and see where you feel more comfortable and what the commute is like. I had tried another aerodrome north of London, which, whilst a nice place, was a 1 hour drive one way if there was no traffic, so the commute time decided it for me.
Hope that helps and if you go for TPC, I may see you around.