some people can do accents, some can't. some people loose their accents some don't. the same goes for actors.
yes there a large number of Aussies in Hollywood who can do good American accents, like heath ledger, and hugh jack man. some like Anthony la pagila and mel gibson can't even speak aussie anymore. some like old rutsty, yes i know he's a kiwi, but he call Australia home, don't even try to speak american. there is a reasonably famous singer in oz, jimmy barnes, who came out here when he was about three years of age, and still speaks with a noticeably scottish accent
from personal experience. almost nobody can tell that i come from Germany. i usually get south africa, on new zealand, both yuck, or even no accent depending on how tired an/or emotional i am. my brother doesn't have an accent at all. i have a friend who came out to this wide brown land about the same time i did, who still has the thickest german accent going, and will never loose it. i know any number of people from englad schotland, canada, and the usa here who have been here for 20 or even 30 years who will never loose their accents.
if need be i think i can do a decent american accent, just give me two weeks over there. a northern english accent could be a bit more difficult, but i've had a good 20 years to practice that one.
so in conclusion. some people/actors as some actors lose their accents, or can do accents, and some never lose their accent of origin, and can't do other accents. granted some accents are more difficult to achieve than others. dc has a point. most people in the english speaking world given a little time can do a more than passable mid western american accent, as yes we have been exposed to it for so long.
the argument is dead as he's a pom anyway, so he shouldn't have too much trouble portraying an Englishman.
