How about somewhere else??
You could finish your PPL, get your ATPL exams out of the way, head off to EFT in Florida and get all your stuff done over there for a lot less than OAT. I did my groundschool at Oxford (well, distance learning) and while I was impressed with most of the teaching I decided against doing the flying there. It just seemed far too expensive for what they were offering and even after I enquired, they didn't seem interested.
If you're from Manchester you'll be spending money on accommodation anyway, so you may as well go somewhere where you get (on average) better weather and spend less. You will probably be finished sooner so your accommodation bill will be a lot lower. Flights are cheap too. Plus, even though FL is a little on the flat side, is is a good place for flying training and there are loads of things to go and do if you want to take a short break from flying.
You mentioned being cabin crew for a few years. Why? You really need to get into the City or something - perhaps offshore....no idea...just get as much money as you can.
Maybe you can tap your parents for the cash? You're 16 so you've got plenty of time to pay it back!!