Hmmm dunno - there seems to be rules and then rules
I always thought of a 'Retained' rank as being officially allowed to use that rank in retirement,and usually would be the highest acting/tempy rank achieved during Air Force service.
Kings Regs [
KR&ACI] (1943) states.....
3647 Resignation
An officer who resigns his commission will not retain any air force rank, except by permission of the Air Council under directions which H.M. the King may be pleased to give. It will be competent for the Air Council to give or withold this privilege.
So obviously John Cobb did not upset the RAF Hierarchy by resigning his commission midway through a global conflict - as the Air Council presumably gave permission for him to retain his highest achieved rank.
I have always thought of an Honorary rank/title as usually being one which has not previously been achieved in the services.