I would argue that at 15 you are actually in a better position now to learn to fly than you will be for many years. This is because at the moment your parents may not be helping you with flying but they are paying for your rent and food (I presume!).
If you wait a few years and go off to university or get a job you will probably have to cover these costs yourself so will find it even harder to save for flying.
So exploit the position you are in now, because pretty much every £ you earn now can go on flying.
If you work just one day a week and get £45 or so, assuming you work full time in the holidays you can make £3000 a year.
So you should have your license by the time you're 18.
There is no secret to learning to fly on the cheap. It's as simple as get a boring job and work it until you feel you have to quit, then work it some more, and before you know it you have your license!