I'd say go to university first. I found university a really rewarding experience and you have far more opportunities to 'broaden your horizons' than you would getting tied down to a fairly unflexible career path at a young age.
Putting the financial debate aside, the chance of exchange years abroad, long summer holidays to do what you wish (travel, charity work etc...), the wide circle of friends you'll make who will end up in a diverse range of careers, etc etc etc..... are all opportunites you will get if you choose to go to university.
The fact your asking the question suggests your not completely 100% sure you want to start a career as an ATCO at this point and your curious to experience university life- so have both and go to university first!
Also, you can get through university with minimal debt if your careful with your money- don't blow your student loan on drink and rubbish- get a part-time job to pay for this (which will again widen your appreciation of other career paths), and if you are able to invest your student loan do it (you won't get a chance of a financial loan on these sort of terms ever again and your going to have to borrow money at some point in your life!).
Anyway, I'm preaching a bit now but hey, that's the path I took and I wouldn't change it for the world!