Another vote for TPC
Family run business, good regular instructors who are there because they love flying and not because they want to get hours and move onto big jets.
Very friendly, and priced right.
Their PA28s are some of the most "modern" I've seen at London flying schools too .. Well maintained obviously but just "tidy" too.
Good advice to try lessons with both schools, but if you're set on learning at Denham then you won't go wrong with TPC.
The other alternative I'd recommend is The Great Circle up in York. Adam does a comprehensive Fast Track PPL course and he is an excellent instructor both for ground and in the air, and Full Sutton is a nice strip. I completed my PPL with them (before I discovered TPC) and ended up doing my IMC with him too.
It depends if you want to get it all done in a few weeks, or spend a few months /years(?) doing it at a slower pace..... ??