Make sure they have actaully got vacancies!!! Sounds stupid but its happend... to me!! I went through just as the doors closed as the defence cuts took hold a few years ago. Selection went perfect, no jobs, but still put me through all the crap.
As we predicted then in the bar one eve back in 2003, the backfire to the defence cuts has come, and now they are screaming for Officers/NCA. Get in there while you can.
Good luck.
ps all the prep in the world wont get you anywhere. Your face has to fit, simple as that.