I wish I'd asked the question when i was 17!
I'm 22 and just finishing a degree in aerospace engineering. My first flying was as an air cadet in a glider, then 20 hours of RAF flying scholarship. I was in a UAS for 3 years and did about 70 hours with them, following 20 hours of RAF flying scholarship which was awesome. Fantastic, highly experienced instructors, and i even got paid for it (to an extent)! If you do go to uni, and it sounds like you're bright enough, its awesome. i've grown a lot, and although flying's still all i really want to do, i've at least seen what's out there and made some fantastic mates - and there are always plenty of girls around too ; )
As well as the UAS flying (aeros, IF, nav (including Low Level), formation, all dual and solo as well apart from IF) its an awesome social scene as well. The RAF dont expect you to pay for medicals etc so you can get an idea of if you are medically 'ok' without spending £400. It's also a good chance to experience competitive interviews and selection procedures. And I don't know, but i presume that RAF medicals are at least as stringent as CAA ones. And, of course, you'll find out if you're any good at flying!
I've looked at flight training schools and it seems that any way you look at it you end up coughing up up to 60k - as redsnail says. Even CTC McAlpine, who train for (i think) Easyjet and Thomas Cook may seem like 'sponsors', but effectively you pay back any training expenses over the first 6/7 years of work out of your wages. The bonus, i suppose, is a pretty definite job at the end of the training - imagine taking out a 60k loan, doing the training to get your frozen ATPL then not being able to get a job. You could end up flipping burgers for decades to try and make it back! Another reason to get a degree/qualification i guess.
All i can talk about is my experience, and i hope this helps.
keep asking questions and learning from other people/their mistakes.
You don't sound too keen on the RAF, and it is a lifestyle so don't do it if you don't want to. But the UAS is non obligatory for the first 2 years - it's a 'try it, you might like it' thing.
anyway, time for me to stop blithering
good luck!