Stu, the best reason for choosing the RAF over the AAC or FAA is that of opportunity - depending on your ability and aptitude you could fly helicopters (obviously everyones first choice) in either SH or SAR roles, multi-engine (hercs, nimrods, tristar, VC10, C17s,A400?) or fast jet (Tornado, Harrier, Jaguar,Typhoon). Neither of the other services will offer you such a choice.
Most of the advice you have been given re the AAC is true but do not enlist as an NCO (just how much do you want to be f88ked about?) and definitely do not join another regt with the idea of becoming an E3 officer or nco - the AAC is becoming more and more 'true blue' with the advent of Apache and far fewer E3 chaps are being transferred.
Unfortunately you might have left it a bit late although I don't know if the age 26 limit on starting pilot training is still extant. Good luck.