You might consider doing what I did. Apply to university to follow my second choice career and apply to NATS at the same time. Even if NATS are recruiting, it can take several months to be offered a confirmed place on a course.
Recruitment processes are a bit of a lottery so you might not be successful first time. Then, as some of the others have said, you could go to university and review your options in the final year.
In the mean time I suggest you start building up evidence of motivation such as arranging a visit to a control tower or ATC Centre. Do some research into NATS as an organisation. It is amazing how many job applicants know next to nothing about the organisation they want to work for. Reading up on selection test does and don'ts and interview technique could also be useful.