A levels all the way mate, you come to a NATS interview and when asked what are you doing right now, or have been doing and you explain you dropped out of your A levels, then not really going to sell it to me that you wont drop out of Air Traffic when the training is tough. I'd want to see an individual that was dedicated to achieving something with their life, and taking all opportunities to do that, so even likely to ask you what are your plans if you dont get accepted, and would like to see that you would be saying Im applying to university and will do that until I can apply for NATS again. That way I would see that you are dedicated to getting into air traffic, but at same time you are being realistic and ensuring you better yourself while you are waiting for that to be achieved.
For your info I have A levels, dropped out of Uni on getting into NATS (i applied as soon as I could at 18 just ended up starting at Uni whislt completing all the selction tests and interviews and medicals etc) but had I failed at any point I was in Uni to have continued with that.