This puts me in mind of a reference to Militia and Regular officers in Upper Canada during the War of 1812. It was very easy to tell them apart at social gatherings. If a man was in red, he was a Militia officer, if dressed in civilian clothing, he was probably with the Regulars. The idea being the former is out to impress others, whilst the latter doesn't feel obliged to wear his "working clothes." This was, of course, not in the UK itself, and once the war ended most of the militia officers would lose the reason for wearing red.

However, there may be a degree of relatedness in the English situation. I can see how for a group of visiting militia officers, scarlet would be the colour of choice at any occassion, where impressionable young ladies would be present. (Trolling for fish, as it were.) Colonels Brandon and Fitzwilliam, and the General, are already known in the neighbourhood. Nor were any of them actively in search of a future partner, or out for a bit of harmless flirting with the locals. Finally, none of them are likely to be "on duty" away from the garrison. OTOH, the militiamen in P&P might have just be released from whatever duties they had been doing.

In the adaptations, of course, red is a wonderful colour on the screen. In P&P, it helps sort out who are the military men. It might also be considered less "confusing" if they are not switching back and forth. (That is, the audience might not get that it would be acceptable for the officers to switch into cilivian dress, and wonder if they had left the army/militia).

A side point, there are a few odd things about the officers in P&P2. One is the lack of plumes on any of the caps (shakos) the officers wear. There's also a few points where I wonder about the propriety of carrying swords in certain situations. (It's been a while since I watched the video.)

As for "Persuasion", it's also been a long time since I've seen the adaptation, so I can't really comment. However, it would again be in consideration of whether or not they had any duties to perform. In some cases it might also be a question of whether or not the individual had a money to buy a "good suit" of clothing.

In short, the military men would, most likely be in civies in most of the scenes, unless there was a definite reason for them to don a uniform. It's just that uniforms make much better film.

I remain,
Jason E.