IIRC, the American accent is also specifically derived from the West Country accent, which to this day has some features that are more associated with AmE such

IIRC, the American accent is also specifically derived from the West Country accent, which to this day has some features that are more associated with AmE such as rhoticity and the father-bother merger. This was because Bristol in particular was a major sea hub in colonial-era England, much moreso than London. As an interesting side note, Americans know the West Country accent as the "pirate accent" thanks to the 1950 adaptation of Treasure Island, where Robert Newton played Long John Silver and managed to convince the director to let him use his native Dorset accent because he thought it would be more historically accurate than the RP accent the director originally wanted him to do, and his portrayal was so iconic that pretty much every pirate story since has used some variation on Newton's accent.