If you're going to uni (full-time education) NHS still cover you.
If you're definitely going to spend that money to fix your jaw anyway, better do it now. Tbh you should have done it sooner, it's easier when you're young. I have misaligned jaw as well, and my dentist said I only have 80% chance of making it 'perfect' (I was 18) while if I did it when I was 15 it would be 100%. Also if left until late it will become much harder to fix and can cause serious problems.