Adam, you are still growing!...your body is likely tochange appreciably in the next couple of years!- that includes your eyesight, The fact is, a medical before you are physically mature, will be worthless by the time you actually need it for administrative purposes. the only plus-side i see, is if you have a latent problem which the exam picks up.
IIRC, you cannot hold a UK PPL under the age of 16, but that does not stop you training!
I know a farmer's son, booked his Tractor-driving test for his 15 th birthday (youngest legal age)...With about 7 years experience of driving everything from an excavator to a land-Rover, he passed!
I make the assumption that money is not a problem?...NEVER pay up front, any more than you can afford to lose!...even well-established flight-training organisations go bust with monotonous regularity.
I would suggest that, as you are unable to actually sit your PPL until you're 16 , you will have time to kill when you have completed your formal training ....to keep you up to speed, you could continue with "lessons"...that is to say, although flying solo (No pax allowed with an unqualified pilot not accompanied by an instructor) You would be under the instructor's control and also paying the higher "instructional" rate of hire.
Buy the textbooks, read them , learn!....some stuff, like the physics of flight, weather, navigation, will tie-in with schoolwork.
Get a logbook! (or make one!) All your "trial lessons" provided they are properly entered and signed-off, can count towards the hours required to qualify to sit the General Skills test. (the "practical" )
Now, consider learning on a 3-axis microlight...often a modern one is a slightly lower-powered, larger-winged version of a "proper" aeroplane.
much lower costs and some hours will count towards converting to a "proper" PPL....you have a lot to learn, but a lot of time to learn it in.....lucky person to have the opportunity!.