That’s my take too. And I’m British.
Excepting the example in the OP, kids do tend to have American accents, though. That I do find jarring occasionally. I don’t mind if a family’s accents don’t match completely but, absent in-story reasons, I prefer them to at least be from the same region.
Seriously. Would Rome or Gladiator or HBO’s Spartacus be more enjoyable if-a everybody talk-a in-a sentence fragment like-a this?
Related question. Did the ancient Romans not have pronouns or something? In the HBO show Spartacus, they always talk like “gratitude” instead of “I would like to thank you” or “deal inserts cock into ass!” instead of “I am getting fucked up the ass by this deal!”
giggle That’s a *great *description, actually.
I tried learning it for my first, zealous, year working at an Elizabethan Renaissance Faire. In fairly short order, I was back to “stage British” instead. 
There’s no “French” accent, just as there’s no “English” accent.
People understand that a cockney accent indicates a lower-class person, while RP indicates someone who’s from a higher social class. I’ve head that much of Monty Python’s humour is lost on Americans because of the different accents they use, and the different stereotypes associated with those accents. (Try comparing Boston and “hillbilly” accents, for instance.)
Would you be able to distinguish between, say, a Provencal accent and a Parisian slum-dweller’s accent? How about Breton or Bordelais? Would you know the stereotypes associated with each?
For most audiences, it makes more sense to go with English accents with easily understood connotations.
Nor will you if the directors want their films to be comprehensible in the rest of the Anglosphere.
The questions seem sort of weird, at least given the more recent examples given by the OP:
The Borgias cast is largely from the UK, and Rome had not only a largely British cast but was also partially produced by the BBC. They sported british accents for the same reason most people on American sitcoms have American accents. It didn’t have anything to do with trying to be “foreign”.
Ditto Hugo. Almost the entire cast is from the UK.
Ditto Enemy at the Gates.
I think the question is “why are there so many movies where the large majority of cast members are from the UK, given that the UK is a relatively small part of the Angloshphere”?
I’ve never seen Tin Tin, but all the members of the cast listed on the wikipedia page are English as well.
I’d agree. Plus the class level to an extent. The little cocky Cockney lad is a different character, than the nicely brought-up lady…etc. Okay, different accents.
There are accent things in the US that have stereotypes as well. The thuggy New Yawker/ Jerseyite. The inbred Southerner. The Texan. Hell, Florida has its own accent as do most states.
Or the no accent ‘accent’ like Carson had. Or the Mid Atlantic.
Das Boot was unusual in the original cast did the dub.
And all those British accents in Star Wars.
Not just other countries, other worlds. People from Westeros and Middle Earth tend to have British accents too.
Of the first twenty actors listed on the wikipage for Game of Thrones, 16 are from the UK. They don’t have British accents to sound “foreign”, they have them because they’re British
Ditto LOTR. Of the fifteen actors listed on wikipedia for Fellowship of the Ring, nine are from the UK (though the Welsh Rhys-Davies adopts a Scottish accent because all Dwarves on TV are Scottish for some reason)..
Again, I think what’s going on is that when British actors are in something that explicitly takes place in the US, the effect an American accent. When they aren’t, they don’t. Because people don’t realize how over-represented people from the UK are in the acting community, it makes it seem like they’re adapting British accents when they’re in flims that take place out of the States, when really its the opposite.
One of the hens in Chicken Run had a Scottish accent, and the American rooster wasn’t even sure that she was actually speaking English. (Oddly, that rooster himself had a slightly Australian timbre to his voice ;)).
In Hunt For Red October, the Russian captain had a thick Scottish accent. 
That’s because he was supposed to be Lithuanian, not Russian. His XO, who was supposed to be Russian, had an English accent.
Sam Neill, BTW, was also in that horrible ABC miniseries Ameяika back in the '80s (which has, blessedly, been all but forgotten). The scenes in it where he was supposedly speaking Russian were farcical.
I remember reading an article back in the '80s whose main thesis was that to the American mind, *British = Evil (going back to the American Revolution, one would presume), or of not evil then “pretentious and foppy.” It cited Vincent Price (born and raised in St Louis, MO) as “the exception that proves the rule.”
As for Monty Python, I agree that a lot of the humor is lost if you have no knowledge of the regional (and class) accents in the UK. My understanding and appreciation of it was greatly enhanced after I’d spent 14 months living in England and Scotland.
FWIW, I once landed a role in a Shakespeare production by being able to do a “posh” British accent, even though I normally speak quite differently. (The producer/director was an upper-class English woman.)
*I leave it to the reader to judge the validity of this thesis.
Because theatre/film is a huge industry in the UK. The ‘creative industries’ (Which includes design) are second only to banking in London, for example.
Maybe he is using the same Egyptian/Spanish accent he used in Highlander?
Well, I can’t speak for Westeros, but Middle Earth is in Middle England (the West Midlands, to be precise).
So they should really have brummie accents.
All the examples in the OP are movies set in Europe, which is still more reason to give them British accents, as that’s the primary European accent of the English language. An English-language movie set on a different continent might be different. I just watched the trailer for “Memoirs of a Geisha,” set in Japan, and the actors don’t use a British accent but a strange, Asian-inflected version of American English.
Likewise, “Death and the Maiden,” set in South America, has actors using American accents.
One European-set movie that’s an exception is “Amadeus” in which at least Mozart, his wife, and the Emperor have American accents.
Frankly, I think more of these period and foreign dramas should use American accents. For the American audience, at least, it’s a refreshing reminder that these are supposed to be real people you’re watching on screen, not stage caricatures. When I first saw “Amadeus” it was a revelation to hear American voices coming out of people wearing 18th century clothing.
Again, I think those are just the normal accents of the people involved. Most of the actresses in that film were born in China and speak English with a Chinese accent even when not acting. In interviews Ziyi has more or less the same accent.