I think you are wrong about the luvvies not having a clue - if you watch any period drama such as Pride & Prejudice a lot of time and money is spent on getting the clothing correct in style and wear. What the luvvies do is show their contempt for the armed forces by deliberately wearing the wrong kit or right kit worn incorrectly.
As they have wardrobe departments who spend mega bucks on research there should really be no excuse for actors playing servicemen/women wearing kit incorrectly.
Never thought of it before. Fair point Croft and Perry were known to point out things that were wrong on set and Michael bates was a major in the Brigade of Gurkas and served in Burma in WW2 So a few people could have pointed out a flaw.
Windsor Davis also did national service.