Hinted at by http://www.quora.com/Did-Americans-in-1776-have-British-accents
“American and British writing has always been and continues to be a common language. Except for a few idioms and typographical conventions, it is impossible to tell whether a writer is American or British. There are more differences between the styles within each tradition than between the two traditions…
the degree of divergence varies inversely with the degree of importance of the subject-matter. That is, where the ideas to be expressed are trivial or facetious the two vernaculars differ so widely that they may almost be said to be foreign languages to each other. When the subject-matter is purely practical or commonplace, the divergence, though noticeable, is of secondary importance; and, finally, when the subject matter is of the highest quality, being concerned with ideal values and fundamental concepts, the divergence is so slight as to be almost negligible.
Although it may not be possible in a collection of formal written documents to discern, except by spelling, which are by British writer and which by Americans, it is easy in any gathering of speakers to distinguish the British from the American. The pronunciation by which British speakers are distinguished is Received Pronunciation, Oxford English, Public School English, BBC English, or standard British English, as it is variously designated.
Received Pronunciation developed at the end of the eighteenth century, during the period of the American Revolution. At that time there was no pronunciation by which people in America could be distinguished from the people in England.
In the eighteenth century, British society began to shift from caste determined by birth to class determined by wealth and occupation, and tools began to be provided for upward mobility. London had long been the political and cultural focus of Britain, so the language of London was recognised as the prestige dialect.
London grammar and lexicon were propagated by grammarians and lexicographers … London pronunciation became the prerogative of a new breed of specialists – orthoepists and teachers of elocution. The orthoepists decided upon correct pronunciations, compiled pronouncing dictionaries and, in private and expensive tutorial sessions, drilled enterprising citizens in fashionable articulation… John Walker introduced the term “Received Pronunciation”. London pronunciation, he wrote, “is undoubtedly the best… that is, not only by courtesy, and because it happens to be the pronunciation of the capital, but best by a better title, that of being more generally received.” .. ‘Received’ in the sense that Walker it, means, “Generally adopted” or “approved”.