Unless you actually want to be an RAF pilot, then don't go in the RAF just for the sake of it. They'll notice straight away you're not committed to them and won't even give you another look. Even then, you're not properly qualified to fly commercial jets even after being in the RAF. The planes are 2 completely different things. And if you do get into the RAF, they'd want at least 8 years out of you before you decided to leave.
If you're really, really serious on working in the aviation industry, you might as well see if you can get your PPL (Private Pilot License) now whilst doing your A-Levels, it'll cost you around £7000 but it's good to get your first foot on the ladder and you can see if it's really for you. If not, keep it as a life-long hobby. Nothing has been wasted. If you can't afford it, don't stress. Not many teenagers can.
After your A-Levels you have the options of going to a dedicated flight school such as Oxford Aviation Academy or CTC Wings, but their integrated courses claim to cost around £70,000 without accommodation and living costs. But I'd always add 10-20% of the 'cost' because in realistic terms, flying always costs more as there are many factors which you can't control, weather being a main one. With that in mind, I wouldn't be surprised if you're looking at around £100k once all costs have been taken into consideration. Also, loans for such courses usually only cover up to half with a property being used as security - scary thought! Doing an integrated course will gain you your CPL and a Frozen-ATPL which means your basically set up to finding a job with the airlines. This route will probably take around 18 months, maybe a little less.
You can also look at doing the above, in a modular way. At your own time and pace and is usually cheaper. London Met University offer a course to get your APTL licenses and there is also a work from home company called ProPilot which do the same but be warned learning ATPL from home, on your own is incredibly difficult and is really not advisable unless you have a passion and knowledge for what you're doing. To get your ATPL this way (providing no re-sits are needed) will probably cost £3000ish but it is a lot harder.
Then you can go to somewhere like Stapleford which offer great courses to get your CPL, this will probably cost you £21,000 - £25,000. Then you'll need your IFR etc which will cost more. Modular, if done right, can probably done for around £50k but it's a lot more difficult. You could always look at doing it abroad which would be slightly cheaper but with a weak pound, it's not as cheap as it was so that should be considered carefully too. And you'll have to convert your licenses once you're home.
If you're very lucky, you may get onto a Cadet Scheme with an airline such as BA or Monarch but these require a £80,000 up front cost which will then be paid back in your first 7 years of working for them.
It guarantees an airline job and type rating which is very rare in this day and age. However, BA, who is doing the biggest recruitment drive only has 400 places over 5 years on this scheme and competition is scarily tough. Thousands apply and the process is very rigorous, so you have to be passionate and know your stuff. Aviation isn't just a job for people who 'don't know what to do and being a pilot looks cool' you really, really have to want it and if you do, that's great! But these schemes are rare and very difficult and people should never assume that they'll be able to get on one. Also, degrees are becoming a favourite for airlines training and recruiting through cadetships.
The next big thing once you've crossed this monumental and expensive hurdle is actually finding a job. There are barely any out at the minute, especially in Europe. There are so many airlines in financial turmoil, like one of the biggest in the world for example, American Airlines. It wouldn't be surprising if you had to move to the Middle East or Asia to find your first job, but they, unsurprisingly favour people from their own country before anyone else. The aviation industry, similar to the rest of the economy is not set to get any better either. But it's very cyclical and things can change in a blink of an eyelid, they can go from being bad, to great, to being awful again. I know many unemployed pilots and some who have had to move to countries such as Qatar and Azerbaijan just to find a job.
Another thing you'll have to look at in the mean time is your Class 1 Medical, these cost around £300 initially and I cannot stress enough how essential they are to get.
As long as you're 100% sure being a pilot is for you and you're committed as many aspiring pilots have been since their age was in single figures, you'll be fine. But you have to want it as wannabe pilots are just as competitive (if not more so) than Med students, it's a dog eat dog world. But the reward is worth it!
I'll leave you some links below that may be of use to you too:
http://www.oaa.com/http://www.ctcwings.com/http://www.balpa.org/Home.aspxhttp://www.airleague.co.uk/http://www.flysfc.com/http://www.londonmet.ac.uk/lmbs/subjects/aviation/residential-atpl.cfmHope this helps and hope you find what you're looking for