I will say though, I think some concession should be given for accents. Like the Gray-um versus Gram pronounciation. I have a really hard time saying it the English way and my mouth just doesn't say Gray-um. I am not trying to be rude, it is just my accent. But that is soooo different from Marsha versus Mar-see-uh. Those are just clearly different words.
I totally agree!
As somebody who's spent the better part of the past 20 years eyeball-deep in British literature, music, film, and general pop culture, I get the colloquialisms. I know all the slang, and I know, for the most part, how things are pronounced.
But, with a few exceptions, I tend to stick with my Americanisms. It's not because I don't know any better, nor even that I
can't pronounce things in the 'correct' way. It's just that, when I try, I feel ridiculous! It seems like such an affectation. It sounds completely wrong and unnatural coming out of my mouth, with my Wisconsin accent.
Code-shifting is one thing. Most people do that, and of course I speak slightly differently (both in accent and vocabulary) when I'm hanging out with my family than I do in a business meeting. But they're relatively subtle changes.
When I try consciously to use British slang or pronunciations, I feel like a pretentious git. Except I'd never actually describe it that way, because doing so would make me sound like an a$$hole. :p