I can't comment on the Cabair operation at Denham - but I can certainly recommend The Pilot Centre.
The training costs (assuming you fly at the discount rate) would be £115/hour (£75 aircraft rental + £40 instructor) - assuming you are not too big to fit in a 152.
Once you have your PPL I think you will be pushed to find anywhere around London that rents aircraft for less - that's why I went there initially (and I'm still there 3 years later).