In America they're generally only aware of 2 British accents (even though there are possibly hundreds), the upper class version and Cockney. As Mitre's character is meant to be very different to Sean they've clearly been told to exaggerate in order to display that contrast. It's always amusing though when the Brits use the American version for some of our expressions. You know, let's not have them be "too" British ;-) For example the other week Sean said something about being on "POTUS" duty - nobody from Ireland of the UK would say that as it's not an expression we're familiar with or ever use - after the show I had to Google it as I'd never heard it before in almost 40 years,
You can't totally blame the writers for it though, I get the impression in the majority of American tv shows where a British character is featured as long as they sound 'British enough' then who cares how bad they sound to people who will know better. Plus Irishman Brían F.O'Byrne is the one actually creating Sean's "accent."
For any Americans who isn't "getting" our criticisms of the show's Brits, just imagine a UK show where all your country's characters have accents that none of your countrymen actually use, wear 10 gallon cowboy hats EVERYWHERE, shout "YEEEHAW" at every point, are 10 stone overweight, threaten to sue each other for "emotional injury" whilst telling everyone "we don't do it like that in the States".....
It's not all bad though,we do call people "wankers" a lot though. The irony though is if one of our own shows used the word as much as they do on TLS it would get booted off the air. But don't worry, we have just as many different swear words and insults as you lot ;-)