The two jobs are quite different. One is a civilian seagoing navigational role, the other is a military aviation role which involves going to sea if streamed in the anti-submarine role, but largely shore based if employed in the Commando aircrewman role.
Flash to bang for RN Aircrew is about 12 months on average but fewer than 10% of applicants who get as far as attempting flying aptitute (CBAT), actually pass it and the follow-up aviation medical examination has a high attrition rate also. Not trying to put you off, just giving the unpalatable truth. Any history of asthma, for example, forget it.
Best bet? Call your nearest AFCO for a briefing: 03456 07 55 55
Good luck.