AB2,
The main thing that you should know about 'making it' in Military flying: you either need lots of ability or lots of dedication, and ideally both.
From your question about your choice, you have to be sure that you have enough of the latter?
The UK mil is going through a lot of changes at the moment, and has a lot of commitments too. Thus, you are going to need 100% dedication and absolute conviction to make it a full career.
While I am about the hop from one to the other, and even though I can no longer tolerate what is happening to the UK military, if I have the make that same choice again, I would still join the Mil.
(>|<)