Ok so yes looks like you can join off the street as an aircrewman. Don't think that it used to be the case (standing by for correction.)
According to the RN website promotion to LH (=Cpl) is automatic if you complete flying training, further (e.g. PO/Sgt) by selection on merit. I don't know how fast promotion is in the branch at present. But there is a big difference between Junglie Crewmen and Pingers in terms of role. Junglies are basically wannabe Royal Marines (if they are not RM already) who sit in the back with a map scratching themselves and winding up the pilots. Pingers are wannabe back-on-board-for-another-3-course-dinner who sit in the back with a load of screens, computers etc looking for subs/smugglers/ships etc who don't wind up the pilots because they can't take it and might cry.
Time at sea can be great but if you are not so tolerant of people in relatively close proximity then give it a bit of thought. i.e. would you be happy sharing a room with 5 other lads, some/most of whom are on a different shift pattern to you and working in different areas of the ship? Some great times but some people love it and some not so much.
Flying around in a mil helicopter is completely different to civ flying (in a lot of ways better, especially for the young [at heart]. Good luck.