BenO,
Just go with your heart - you sound level headed and I would be surprised if you were seeking a life/career decision on PPrune posts!
In my humble opinion, it will soon not matter and you will be serving as part of an air wing of the UK Defence Force - one uniform, one single service and one career.
Whether right or wrong is immaterial - finances will sadly force you and I to this inevitable conclusion (which may stop the puerile willy waving in previous posts and actually unite us as H M Forces against the Treasury and MoD civil serpents that actively try and 'divide and conquer' for their own gains).
Just do it - work out what you want to fly, and predominantly in which environment, and if you are good enough then the world will be your oyster, and as alluded to above - nothing is fixed and if the demand in a particular area/Service is not being matched then there will be more opportunities should your preferred choice not come to fruitition.