At the risk of crediting a stupid question with a sensible answer, I’m surprised you didn’t mention the ‘enormous savings’ to be made otherwise why do it?
Except that there wouldn’t be any. Most if not all the personnel would simply change services (£££ for new uniforms etc), Air Marshals would become Generals & Admirals (unless you could successfully argue that those of that rank in the RN & army had sufficient capacity to take on the extra responsibilities, in which case why are there so many underutilized admirals & generals?), ditto for adminers. You would need the same number of airfields, ATC, engineers, suppliers etc. If the aircraft numbers remain the same then you need all the support network as there isn’t much duplication between the services.
So where would the savings be? And if it’s not going to save any money why bother? Unless it’s to satisfy a long held grudge against the RAF?
As the FAA & AAC are so tiny it would make little sense for them to take over the (much larger) RAF. That would be like the RAF Regt absorbing the army. Which probably makes as much if not more sense than the original question.