I was in the same position as you some years ago too. Personally (with hindsight), I don't think I was mature enough to join NATS straight out of school (I would also have had to take a gap year as I went to Uni when I was 17).
I think it really depends on how you feel you would be suited for the course now or later - I definitely wanted to do ATC when I was at school but think I'm better prepared and experienced now after spending time at uni (I've got the 'getting paralytically drunk' phase out of my system, and I'm fine with intense studying courses).
As for me, I've got a MChem in Chemistry and a PhD too...