Originally Posted by
brakedwell
I went from Flight Lieutenant to Captain, but never used either in normal life.
Strangely enough, according to Debrett's, as a Flight Lieutenant you should not, but as a Captain, providing your post was connected with horses, you can.
It seems to go back to when officers of field rank were "laid off" on half pay between wars, so Majors and their RN equivalents were still serving but not in uniform. The exception was the cavalry and RA whose Captains were also on half pay.
There were a few question marks about Mark Phillips calling himself Captain but being ex cavalry, he was quite entitled to do so.