Good aero-eng degree --> organisation, building up knowledge and employer's trust --> persuade flight test to take you on --> make bloody awful tea so they may as well let you try doing some FTEing.
If you want to go rotary, in the UK, Southampton, Liverpool and Glasgow are the good places to have your degree from, as they all cover significant helicopter content.
Most FTEs in P&HQ and mechanical systems tend to be civilians, although a lot in avionic systems and weapons come from a military background. There's no absolute rule however.
Even very occasionally FTEs move between fixed and rotary, although not all that often.
G