I for one won't jump down this young person's throat. Good on you for finding out about a potential career before you have left school. I know all too many final year Uni students who aren't even considering a career, and they graduate in a matter of days!
The way I see it you have the NATS option, which I feel has been described here fairly well.
The Self-funded option - depending on how many ratings you want, even if its Tower Only you're after to begin with, you're looking at well over £10,000.
The Military - They will very much move you away from home, and yes the Royal Navy is by far the best uniform going in Air Traffic, and pay you pretty well for the privilege too. But, for now at least, your ratings will not be valid outside of the military. So when you retire from your stint inside, you will be looking to pay for your courses anyway.
Other options I feel might be worth mentioning are:
Eurocontrol. Not currently recruiting for controllers, but by the time you leave school they might be. (I think the wait is at least a couple of years at this time).
And finally;
It might well be worth getting a job as an Air Traffic Control Assistant and looking for onward training through you airport, once you are there. It is by no means certain as an entry route, but if you can make it happen (and people do), then you'd have the benefit of a job to go to after getting the rating, which gives you a place to validate, a steady income and you may well not have to pay a penny for you training either. The downside is, you might wait a whole career to see this come to fruition.
Hope it was useful,
Phil