Joining before BRNC



I'm currently applying for BRNC.
I'm going for an interview at my careers office soon, and hopefully then off to AIB.
I'm considering (once my A-levels are finished) joining the RMR. Hopefully that will help to prepare me for a lot of the new military experiences (previous experience is limited to 5 years in the CCF(RAF)).
I want to keep time between my first interview and BRNC as short as possible, although I realise it's out of my control. It does sound like it would take 8-10 months or so, which appears to be the same length of time it takes to complete RMR training.
I'm also hoping that it will come across well at AIB, as it's clearly a positive action on my part. The reason I'm looking at RMR over RNR is I feel that if I join the RMR I'll be getting experience of the RM that will help me in a career in the RN, which would serve me better than getting RNR training that I'd just be repeating at BRNC.

I have a couple of questions though.
Obviously, does anybody know someone who's done this before, or something similar?
I feel it's taking the piss slightly to turn up and say "I want to train with you, but I'll be pissing off as soon as my date for BRNC comes up". Is there any contractual period I have to sign up for?
Is this a really dim idea?