Don't put all your eggs in 1 basket.
Choose your A levels and give them your all. Nobody can take them away after you pass them and it'll look good on your CV for any job or CTC application etc.
Think about how you'll save £50k for your modular training too. £50k is going to be hard to save but your 14 and you have time.
Think about what you'd do if for some reason at 21 you can't get a Class 1 medical? What would your second career be? For me if I couldn't be a pilot then I'd be involved in IT ... Only when I was 18 I didn't have the cash for the training so IT came first.
I've built up a few years of work experience / money / my own place and now I'm looking to do my training. If for some reason (economy / no jobs left) I can't find a job as a pilot I'll have my IT to fall back on.
If I had my parents re-mortgaged their house when I was 18 and I couldn't find a pilot job where would that leave me?
Good luck.