A few too many personal axes to grind here,
Going back to the original question - I would feel perfectly safe flying on any UK AOC holder. Don't get confused between the original question and your personal opinions on the viability of each companies business model.
FYI I'm pretty sure that Ambeo were operating before Flairjet.
IMHO the biggest risk risk you face is losing your prepayment, and that's a pretty darn small risk unless you are buying many months worth of flying.