Well good luck with the GCSE's. As for A-level, don't worry too much about maths and physics. I took both and have struggled with physics, therefore only taking maths (plus English and geography) next year. It's probably best to do things which you're good at, and enjoy.
The PPL - this can be done at almost all flying clubs. You firslty go down to one, and have a trial flight. If you enjoy it, and feel you want to learn to fly then you book another lesson. If for some reason you're not happy with the flying school, have a look around some others, and pick which you feel most comfortable.
It costs a fair bit - I've known it be as low as £3500, but realistically its more like £4500+ depending on where you learn. I'm expecting to pay in the region of £7000 for mine. It can only take a couple weeks to get the PPL. But if you plan on taking your time with it (say one hour a week) it'll take at least a year to get through it. There are 7 ground exams, and a flight test to take, as well as being required to take 45 hours of flight training. All of this will be explained when and if you decide to go for it. Have a look at the "private flying" forum, as well as flyers forums. Both have good info. on PPLing. I must say, I'm only 5 hours into the PPL, but it is fantastic. If it's possible for you to do it, do! But it is a good idea to get a trial flight....just in case it isn't for you.
JW.