Hi Jake,
I am only 2 years older than you , just finished GCSE's so generally know where your coming from. I think you should pace your ppl training slowly and pretty much fully concentrate on your GCSE's. Do enough flying just to keep your passion fuelled, but really knuckle down on your GCSE's when the time comes nearer because unfortunately it is true that the exams are becoming more difficult to get those top grades. Get good maths, physics and English Language and just pass the rest and your sorted.
But mate just enjoy yourself really. I've painted a mildly negative picture but if you work hard and gather your info you won't go far wrong.
My 'plan' if you like is to go to university and get a decent paying job. I could possibly gather enough cash to get myself on an integrated course and do it as a career change. I'm not prepared to gamble with money I don't have.
Good luck on your flying and GCSE's