I read somewhere that the American accent is closer to what British was like in the 1700-1800s. Later the Queen's family, which came from a corner of England, established its accent as the norm. So the American pronounciation is original, and the British one changed in the meantime. Not sure if this is true.
There is be a certain degree of irony to English going through the Great Vowel Shift† ( https://en.wikipedia.org/wiki/Great_Vowel_Shift ) and then having a German dynasty which manages to distort the language again††. † Which made English less-Germanic sounding, at least to my ears. †† In my humble opinion modern British English sounds marginally more Germanic than American English.
This should be one of the first things that gets hammered home when learning the first foreign language.