Ross, you're going to find lots of people that will tell you it's fine to do ATC without Uni and then you'll find lots of people who tell you to go to Uni first.
The point is that it can only be your decision. If you have any nagging doubts, then don't get into ATC yet. I personally didn't get into ATC until I was 25 so I'd had a bit of freedom before hand, so I was ready to get stuck in. I did apply when I was 18, but decided against it and never went through the process. I was worried about being tied down to the same job and possibly the same company for 40 odd years. Having that bit of freedom before and time to do what I wanted meant a lot.
I love the job now and can't think of anything else I'd rather do, but if I'd gone straight into it then I might well be getting itchy feet.
Anyway, as I said, others will disagree and advocate going straight into it at 18. The choice is up to you, but make sure you feel as sure as you can be.
Good luck with the decision!