You'd be better of joining the RNR - not sure which unit is neaest to you. Or possibly the TA or other Reserve Forces.
Being a Reservist will give you more of an insight into things and the training will make subsequent RN training seem easier as you will of got some experience of the basics. And of course it shows commitment.
Hopefully you should be able to get some CW training to help you prepare for the AIB.
RNR
A quick tour of the RN website (
www.royal-navy.mod.uk) should help you brush up your knowledge, as will going to
www.rnreference.mod.uk and looking at the Rolling Brief.