Good luck trying to learn British accents - if you succeed you'll be one of a very few American actors who've managed it. The best recent example was Gwyneth Paltrow in Sliding Doors; not only was her accent flawless it was also very well-observed because although her character was quite a middle-class Home Counties-type woman, she flattened her accent into more of an Estuary type voice, which is exactly how that sort of woman would speak. Contrast this with Renee Zelwegger's portrayl of Bridget Jones, a character who is almost certainly supposed to be a girl much like the one Gwyneth played, but Renee hadn't done her research so well or (more likely) was badly advised (by director, accent coach or whatever) and played the character with a rather posh, braying RP type voice.
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