Speedbird
I agree with Timszta and J-Heller. If you join as a flyer in the RN or RAF, you will be an officer first and a flyer second. In the Army you are a soldier, first and foremost, above everything. There are a great many other things that will be part of your everyday life that will push flying further down the list of the day's tasks. It won’t be a case of turn up, fly, and then poke off home - if that is your intention then the forces are not for you. Being able to walk Dartmoor is a small part of what is required - you will be expected to lead men and women in areas and situations you may have thought possible only in war stories.
If you are joining any of the flying forces with a view to joining the airlines, I think you are very forward thinking in mapping out your life. However, don't join the forces and take up the place of someone who wants to fly in the forces as a career rather than a stepping stone to their next job. Joining the airlines is most definitely not in the back, front or any part of my mind and I joined to do what I do now. Circumstances may change and I may change the direction of my thinking, but I am focused on now and my next couple of appointments continuing to fly in the RN.
If you want to be an airline pilot, then apply to the training school and join the airlines. If you join the forces, you may find that the flying and life far outweighs anything you will do in civvy street and decide to stay, if you get that far. Do beware, however, that the man who focuses on the last hurdle usually falls at the second.
Good luck in whatever you do.
