Oddly enough, not even Terry Gilliam. 
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James Marsters is from California.
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His Brittish accent is fake.
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He’s 41 years old(this was always rumored, but has recently been confirmed by Californian birth records).
Yikes!
Probly the best English-to-American accent I’ve heard lately is Janet McTeer in Tumbleweeds.
I think Australians/Kiwis do the best American accents. (And don’t smack me, but every time I hear Nicole kidman’s real voice, I wish and wish that she would affect an American accent all the time…)
It’s actually rhyming slang. “Berk” is short for “Berkshire Hunt”. I shall leave it to you to figure out which insult rhymes with “Hunt”.
Oh–and I think the Crocodile Hunter’s wife does an exceptionally good American accent. 
He certainly is well preserved.
Given the amount of Romani loan words in various British dialects, I had assumed ‘berk’ was another one. Looking up the definition just now, I got a load of hits for the rhyming slang and just a couple for it being Romani. I’ve also just found out that the Romani word is actually a loan word from Arabic.
The things you can learn from the internet.
To continue this mini-hijack:
Just read that there’s an English slang word for breasts - “Berkeleys” (and no, I’ve never heard of it). That one’s meant to come from the Romani “berk”.
Hi-jack over.
Old movie, but I thought Kenneth Branagh’s American accent in Dead Again was great.
For some reason, Americans have a much more difficult time faking Irish and Scottish accents than British ones. Maybe because of the inflections, but I (being an American) cringe when I hear Irish accents that sound like the Lucky Charms fellow.
Irish/Welsh/Scottish seems a full step further removed, to me, from a midwest American dialect than, say British RP. It’s not only learning the mouth sounds, but attempting the singsongy up-and-down rhythm of speech. My impulse is always to overdo this, and I think that’s what you mean by “Lucky Charms”. It’s a shame too, because the accents of the UK have such a marvelous variety and personality. One would like to be able to converse well in all of them.
I can’t quite remember this myself - was… er… (oh I hate me damn memory) the irish guy’s American accent good in …(I refuse to forget the name of the bloody film…) erm…Minority Report?
(I don’t think I knew he was irish at the time of watching that film)
Oh, and my reference point for Bob Hoskins’ American accent was ‘Super Mario Bros’ Which was actually a good film.
Oh and I love the word ‘Berk’. It is sooo douglas adamsey very english word. (not pleased to find out it is part of rhyming slang for ‘cunt’, which is not such a nice word)
I am a brit and I can’t even do that - There was a Geordie at my previous job . I could Not understand a single word out of that guy’s mouth!
Sorry - Geordie = Native of Newcastle and other parts of NE Enlgnad.
Other parts of NE England? Whey ay, mun, ye divna want te be calling a Mackem a Geordie, naa …
I beg your pardon …
I mean that you need to draw a distinction between natives of Sunderland and natives of Newcastle upon Tyne, even though the dialects are quite similar to the untrained ear.
(And, FTR: Alexis Denisof - pretty convincing; James Marsters - good and consistent, but never fooled me; Juliet Landau - her character’s meant to sound like a loony, right?)
I know. He looks about 28.
Unlike Americans who pronounce the can in can’t, Brits, especially the ones with the accent he is trying to pull off pronounces it as carnt, not cant. Other than that, it’s not too bad
How about for 13-year-old zombie Brits?
They pronounce “braiiiins!” as “briiinnes!” 
But only if they’re pickled.