I am NOT ex forces so there is no reason for me to know this but my understanding is that at a certain level you "Carry your rank" into civvy street.
For Royal Navy that rank is Captain, but because Commanders are often promoted on retirement to Captains I think they get it.
I do not know the equivalents for Army and Airforce but if anyone is using the title Captain, then it is navy or its bull5hit (and that goes for ATPL also)
I'm pretty sure that it's actually Sqn Ldr / Major / Lt Cdr level that you are entitled to use your rank in an honorific sense.
The practice if promoting on retirement no longer happens and hasn't happened for as long as I can remember it - certainly in the UK military. RN Captains above a certain level of seniority may be appointed Commodore for a specific posting, but unless that 'promotion' is substantiated they will revert back to Captain at the end of the tour.