What is your objective? If you are just doing your PPL for fun and can only have lessons at weekends (my situation) then it is important to be reasonably close to the school. You will lose lessons to weather and if you've driven 1 hour+ each way it can waste a lot of time. So if you are local to Biggin I'd be tempted to go there.
On the other hand if, as I suspect from your comment about accomodation, you want to get your PPL done in a short amount of time as the start to a wider flying career, then look at a variety of schools, and maybe overseas on their own merits.
I'm learning at Stapleford and its great so far, high quality of training. They seem to take it quite slow to start with, leading up to first solo slightly later than many schools perhaps achieve. Personally I like that, solo will come and I want to get the fundamentals down as well as possible. Can't comment on Biggin Hill never been there.
The exams aren't particularly hard, so far I've done Air Law and Principles of Flight, passed both first time, studing a few evenings and a couple of weekends. Next is Met which I'm having to work at a little more. If you've done GCSE's, A Levels etc then you should be fine self studying.
Enjoy the training!
