Remember the panel will not be looking for someone who's already in the RN, but someone who's interested in it and has done their homework. The RN website has basic info on 1SL, 2SL, CINCFLEET etc - know their names but you don't need to know where they went to school!
Know about current issues in the RN - new carriers, Astute class subs, T45 destroyer etc. Also about big issues in the news at the mo - read the paper for the next week so you can talk for a couple of sentences about the big articles. Have opinions on things - you may be asked something controversial, like what you think of immigration. As long as your answer isn't bigoted or racist, as long as it's been thought through then you'll be ok.