Doesn't the Duke of Kent hold one of the highest ranks in the Army, even out of the RF? I think he is a Field Marshal, a title held also only by Charles and Philip.
He served for over 20 years in the Army until the 1970s.
Many of the senior royals have high ranks. There is the rank they achieved while they served though vs promotions since:
-Prince Philip: -during service, commander of the royal navy. Since retiring, he has been promoted to lord high admiral (served in WWII)
-Charles- lieutenant in the royal navy when left duty in 1976. flight lieutenant in the airforce. Now post retirement, admiral of the fleet and air marshal
-Andrew: retired with the rank of commander. Now honorary vice admiral (fought in the Falklands)
-Duke of Kent: Royal Scots Greys. Promoted to lieutenant colonel in service. Field marshal since retirement (peace keeping including Ireland and Cyprus)
-Prince William- captain in Blues and royals, flight lieutenant RAF while serving (search and rescue RAF) Major in army, squadron leader in the RAF since leaving service.
-Prince Harry: Captain in the blues and royals (deployed to Afghanistan). Since retirement a Major in army, squadron leader RAF
-Prince Michael: Army in The Royal Hussars (peacekeeping) retired as Major. many honorary appointments since
extended family:
-Earl of Ulster (heir of DOG) Royal Hussars (seen duty in Northern Ireland, Kossovo and Iraq) he retired with the rank of Acting Major.
-Tim Lawrence: Vice Admiral of the royal navy (Northern Ireland among other deployments)