>a) most are in British locations

No they ain't.

>b) if they aren't, they're likely still using classically
>trained British actors to match the "classical sounding"
>dialogue

That's it though, why do you need british accents to deliver "classical sounding" dialogue? That's a supposition someone made once but it doesn't make any sense. If your movie's in ancient greece why would british accents give the dialogue more weight?

>c) they aren't using British accents in every single period
>piece ever

They are in far more than make sense.