Mikek16,
I remember that bit of the application confusing me a bit too, can't remember what I put in it (if anything) but I'd just leave it blank if nothing comes to mind. All you need in terms of qualifications are GCSEs - I think it was 5 at A*-C including Maths and English, but might be worth checking the NATS website just to confirm that. I think this part of the application is just for extra qualifications and things such as membership of professional bodies or institutes. There were a few other really wierd bits of the application, like the section for your "skills" which was really confusing, but I don't think these parts matter too much (as long as you have the minimum qualifactions required acceptance, as far as I am aware, is based solely on your performance at the selection stages).
You can always email NATS HR if you're really worried and want some guidance.