Would you be allowed to make the frame jig (mostly bolting, welding and cutting which it sounds like you're already capable of) this year and build frames for you as and a levels? Spread the project out over 3 years and you'll probably end up with a much better finished product? Could learn to braze in your spare time then by next year(assuming you're at the same school/college) your teachers will understand your project a lot better and see your skillz and will probably let you braze cos you'll be older and wiser and you'll get to know them well this year with the jig.
Sounds like you're totally keen, just don't risk your maths and physics grades this year if you're serious about engineering. I went from potential engineer to guitar making course to farm laborer to tree surgeon and now working on bikes on the side and wish i'd had the direction it sounds like you've got. The path to randomness i took i can pretty much pinpoint to spending shitloads of time on my a-level dt project and doing crap at my other subjects. Bad a-levels = lots of good uni courses closed to you.
If you can work your nuts off you'll be fine, sounds like your dads workshop is pretty kitted out so could you do the work at home and still get the marks?
Hey Nice advice from everyone again, edmundro - i wont be taking DT for A level as the college im going to doesnt do it (the school i am at does have a 6th form, but i hate it here, cant wait to get away from shitty private schools. . .) at the moment physics teacher is complete bollocks so im teaching myself, yet still predicted an A*. . . man these tests are easy, cant wait till A-level when i can do something challenging and interesting, also not allowed to make anything at home, everything has to be supervised, ive decided im going to make a desk now :/ i need one, so might as well make one to my spec :) im not going to give up on frame building though. . . im guna keep an eye out for brazing kit on the cheap and in the mean time read a hell of alot, you can never read to much when doing stuff like this :)