When an English language movie is set in the past in a non-English language country, formal British accents or mid-Atlantic accents are traditionally used. The more formal speech helps set the tone and creates an immersive feeling that this isn’t taking place in the current time and place or vernacular.

The Google AI summary actually does a decent job of citing the different reasons since this question has come up before. Do a search for something like “why do modern movies use English accents to invoke the past”.

Anyone remember Kevin Costner’s American accent in Robin Hood? Now it’s a whole movie of Costners!

[–] Lumidaub@feddit.org 11 points 5 days ago

Kevin Costner’s American accent in Robin Hood?

Exceptionally bad example using an English folk hero.

Robin Hood (Cary Elwes): Unlike some other Robin Hoods, I can speak with an English accent.

[–] roofuskit@lemmy.world 6 points 5 days ago* (last edited 4 days ago) (1 children)

Yes, I get what the tradition is. But the tradition is equally silly. Substitution of one farce for another.

Just because that's the farce they always go with doesn't make it better, it just makes it what people expect. People objecting because the movie did something different are the same people crying about the cookie cutter tactics of the movie industry are ruining art.

[–] EggInDisguise@lemmy.blahaj.zone 3 points 5 days ago

It's so specifically a farce that has been relentlessly mocked for as long as I've been alive at least.

That anyone is using the "wrong" incorrect accent is laughable.

[–] usernamefactory@lemmy.ca 4 points 5 days ago

The more formal speech helps set the tone and creates an immersive feeling that this isn’t taking place in the current time and place or vernacular.

Those are advantages of more formal speech. There are also advantages to using more contemporary speech - it can feel more accessible and relatable. There’s no one correct approach, it’s a matter of what tone best suits the film.