I'd recommend waiting until you're seventeen, at the earliest.
Start saving up now and putting money into an ISA, so by the time you reach seventeen you've got the funds or at least most of them to pay for it. Its worth applying for the GAPAN and the Air League scholarships, but remember competition is very fierce and hence there is an enormous possibility that you won't be awarded with one.
Another idea is to join the Air Training Corps. There'll be opportunities for free flying, and also the ATC offer a gliding scholarship which is a bit more attainable! It can't hurt to start reading up on the ground-school subjects though, although I'd leave the exams until you commence flying training for your licence.