I certainly wouldn't recommend the forces route, the army even less so. The training is poor now (bare minimum standards) and almost everyone I talk to now can't wait to leave.
Find a good civilian apprenticeship and stick with it. I'd also recommend avoiding any start-up companies that offer apprenticeships. I've seen a few first hand, and they tend to be unstructured, poorly tutored, and quickly die when the company gets into money problems.
Aim for the most prestigious schemes, and see what turns up.
One of our apprentices left two months ago after witnessing how poorly our workforce was treated during farcical redundancies last year. That, on top of the atroscious 'training' led her to leave aviation completely and win a proper apprenticeship at Jaguar/Landrover. Would Lizzie consider a growing industry over aircraft?