You join as a rating, any branch, and once confirmed as a killick (Leading Hand) you can apply, I'm fairly sure, though the numbers applying are fairly high, I seem to recall. As with the AAC there is no guarantee that you'll ever leave the ground.
My advice at this stage would be to try for RAF NCA for starters - 13 GCSEs is almost twice the number I had when I joined, and that that time you could join as a Short Service aircrew Rupert with just five, so long as you could walk and chew gum at the same time
Are you sure? Surely with the introduction Merlin Mk 3 they need to nigh-on double their quota of aircrewmen?
Nothing listed on the RN career website right now. I doubt with the anticipated cuts that the RN will actually see an expansion in Junglie rear-crew, and anyway some of the light-blue NCA may fancy moving to Zummerzet.