Graham
As far as i understand the education system, A Foundation Degree is a higher level qualification than any A-Level.
And specifically in regards to NATS, i don't think it makes much difference as long as you have attempted the studying, I did three A-levels (OK, it was 12 years ago) but i didn't pass any of them. I start my ATCO training next Monday.
Why don't you apply online now and see what happens, if they tell you you aren't qualified enough, you'll be eligible to reapply in a years time, so this will give you the time to take any relevant courses.
From what i've seen so far, i agree that Maths (Statistics) and Physics type courses will do you the most good.
Cheers Pete.