Tobs,
One thing to point out - You assume that you will be earning the same amount at age 16, 17, 18 and 19 as you do now.
Your earnings will increase, and assuming you earn enough for an hour a month now, when you are 16 and subject to minimum wage you'll be earning enough for an hour every fortnight, and it will only increase.
Using this as a base, there is no reason you can't achieve your PPL aged 17 and not the 19 you assumed.
At 15 you are most likely too young to be considered as a genuine candidate for most scholarships (check the conditions and small print. Granted, someone has to be awarded them and no reason it can't be you, but you're more likely to be successful in a few years time).
So, my advice if you want it:
Concentrate on school and make sure you have a social life! You're young, you want to succeed, but don't waste your youth worrying about money, plenty of time for that when you have bills and rent and taxes and cars....
Work and save your money, you might find having a separate account for flying money may help.
Hang around airfields as much as you can, offer to clean planes/ mow grass/ make teas etc. You'll find that as you make friends you'll be invited flying. Even if you can't log it, you'll get experience.
As for the scholarships, don't be disheartened if you don't get them, try and try again, whilst continuing to build your experience as much as you can.
4015