First of all do you want to be an officer or just a pilot? The air force and royal navy will consider you as an officer applicant first, therfore needing the 5 GCSEs and 2 A levels or equivalent before you even apply. Otherwise Have you thought about the AAC? There is an option of Sgt pilot (generally more flying than the officer equivalent, which you will also need the required potential officer qualifications) which you are eligable until the age of 32 (compared to the RAF equivalent of 23, RN 26 and AAC officer 29), which you can have with no qualifications, just a minimum of 3 years of service in any regiment of the army, which is probably a similar amount of time as re-sitting your exams. You will also avoid the sh*te involved in being an officer. Of course this is assuming fast jet flying is not on your agenda. This option also avoids the killing fields of OASC, AIB and RCB, where the subject of enjoying flying is the ultimate taboo! If you fancy the officer life though, the RN is probably the way to go, as they need good officers and pilots (note the order they are in) more than the air force, and there is no option of getting stuck on VC10's and Tristars (apologies to any pilots stuck on those two types who actually enjoy it)! Whatever you think 19 is still young, as the RAF is not the only option, i have friends who had no qualifications decide at a much later age that they want to be military pilots and worked hard to get where they are now.
Good luck
Mac