I don't know of one single incident in which anyone from any airline committed a single terrorist act.
Yet!
If they can infiltrate hospitals as doctors, they can do the same as pilots with airlines. Note, it doesn't have to be an U.S. or U.K carrier. With several pilots working for one carrier it would be only a matter of time before two of them are rostered on the same flight. Doesn't have to be a pax flight, so forget the skymarshal (who with a locked cockpit door couldn't do anything anyway). They could even form their own ad hoc cargo airline and......Oh I could just go on for ever. There's no solution except to try and convince these terrorists that there are no virgins waiting for them.