Chris,
Good luck with your ambition - about 12 years ago I was in a similar position to yourself. I joined the Air Cadets, got loads of free flying including a gliding scholarship, got to go abroad and bags of adveturous training.
I got my PPL through helping out at a local flying school and getting paid one lesson per weekend.
I would suggest either way would definately demonstrate to the RAF that you are serious, (I got through OASC so I must have done something right!)
At 15 you may be a bit young to work at an airfield, most companies will need you to be 16, so perhaps Air Cadets would be the way forward at the moment.