Slight bias here- but it depends what you want to do as to which service you join up in.
If you want to fly for a career, I would recommend the RAF. You'll go in as Sgt Aircrew (paid about £33k after training) and spend your whole career if you wish just flying. You can go in as a sensor op in the ISTAR world hunting for submarines, enemy radars and other stuff you'll learn about in time. Or as a crewman on helicopters in high intensity ops, often under fire and very, very low. AT loadmasters travel all over the world and from what I see have a jolly good time! The best bit is, you can even go in as one and then transfer role if you don't like it as the common training is quite substantial.
The AAC and and RN don't really have career aviators - you'll do time in the ranks before selection to aircrew. Then you'll fly for a bit and eventually be returned to the ranks again. This is especially true of the RN - as they have quite a structured career mapping - but the AAC seem a bit more flexible for NCO pilots who may contnue to fly their whole career.
The best thing to do would be go to the careers office and see all the departments. They won't be offended if you go around all of them on the same day - and they'll give you a better idea of what they are looking for and probably more up-to-date information than this forum can.
I await the flaming from all AAC/RN guys here!