Having said all that, NATS very very rarely will recruit ATSAs from outwith the company. With the regular flow of failed trainee controllers and MSG type grades, there is usually someone floating about who already has a relatively more complete knowledge of ATM and NATS.
My advice would be to either focus on non-NATS units for the experience, or look into doing a couple of 'A' levels or equivalent.. This could something like BTECs, NVQs or GNVQs. Also remember that NATS don't lay down any subject requirements on these at all.
I remember seeing a presentation from a NATS recruitment/HR type buff, and they were stating that at the end of the line validation rates didn't reflect on the individuals qualifications past GCSE level. Remember, all they are looking for is an aptitude towards bookwork, the real test comes in the formal aptitude testing and interviews.