If you are just interested in getting the PPL however you can and you dont want to have flying as something you do each weekend or whatever then the cheapest way is probably to go to America and do the whole thing in 2-4 weeks, although you'd have to convert the license when you got back. It would still work out cheaper though, maybe £3000 and maybe you could get your parents to pay for the plane there and the accommodation on the basis that its your holiday?
Get "Learn to Fly Guide" which is published by Flyer magazine each year and should be out soon. It is very useful and lists every flight school in the UK with prices and info etc.
Some schools offer a discounted price PPL if you pay for it all upfront.
Get a job as soon as you can ( I dont know how close you are to 16 but you can still get a paper round now and do limited hours in other types of work). When you are 16 try to find a job for the summer holiday ie 6 weeks full time or something like that, that's what I did. It ruins your holiday but you rake in the money. Then get a weekend job when you go back to school and maybe you'd have enough money to start the summer after when youd worked full time. Anyway you go about it you'll have to do loads of work to get enough money to fly.
Good luck!