I’m not sure how true it was but an anecdote my social studies teacher told us was that the dialect was closer to victorian/older (not quite Old English) English and that’s how Britain used to speak. However, in my opinion, they probably confused that with Britain specifically changing to non-rhotic English annunciation post the Revolutionary War with the, now US, to further separate culture. I don’t study linguistics so maybe she was right and I am wrong though. I’ve just never happened across anything of repute backing that up.