"England and America are two countries separated by the same language."
Sir Walter Besant
It was the language of an Empire on which the sun never set. It's still the language which holds the nation of India together (the only other things that do that are the railways and cricket), it's the language which Napoleon cited as the "festering wound upon the world's tongue" and without, the works of Shakespeare, Wordsworth and Ogden Nash would have never existed.
Much is made of the great vowel shift in the accent of New Zealand, and of the 44 different dialects of the UK but little is written about how the seemingly obvious and quite startling differences between American English and British English actually came to be.
I start this discourse with the work of Dr Samuel Johnson and his "A Dictionary of the English Language" published in 1755. Various attempts had been made to codify the language before, some even giving lexicography and etemologies but Dr Johnson who had hated schoolmasters and generally was showing signs of turning into a curmudgeon, sought to provide usage notes to what he saw as a language which was both moving and dynamic. Unlike most modern lexicographers, Johnson introduced humour or prejudice into quite a number of his definitions. Among the best known is "Excise: a hateful tax levied upon commodities"
Over in the USA the language carried over by the pilgrims was already mutating. They found that many words were being imported from the native indians and from the French who were busily taking up bits of Canada.
The single greatest shift in American spelling was not by accident and was done as a conscious effort to show that the fledgling states had truly broken away from mother England. Noah Webster published his "A Grammatical Institute of the English Language" in 1783 with something in the order of 4500 different spellings to what was standard and acceptable in the UK. This accounted for about 10% of the book and by the time his dictionary was completed in 1833 the damage was already done. American English would forever cease to be the same.
Back in England scholars were looking to codify the language even futher so that it matched rigid rules for a Victorian prim and proper country. However, from 1858 until 1933 their task was not complete and when the "Oxford English Dictionary" became the standard repository for words, regionalisms still abounded.
The OED should in theory be the last word on everything in English. It does have tracts on American spelling but realises that American is a dialect. Australian English sits uncomfortably between American and British after influences since the end of WW2. There is the Macquarie Dictionary which is supposed to be the definitive guide to English in this country, but its authority is feeble in comparison to the OED which is exhaustive and now rounds out to 23 printed volumes in its 3rd edition (OED3).
I have a copy of OED3 on my desk and as such I like the UN, the ISO and the WTO consider it be the standard. I think I've inadvertantly taken up a chronic dislike of Noah Webster who attempted to bugger up the English language. Until such time as Newspeak takes over (see Orwel's "1984") and Oldspeak becomes crimethink, OED3 will continue to be my weapon of choice and as such "a good thing" (no apologies to Sellar and Yeatman).