This is all a bit oversimplified. Visit different parts of the UK, and you’ll find that pronunciation and accent vary by region and always have. Not everyone who immigrated the US or Canada came from the west end of London. It’s probably quite rare to find any 3rd generation or greater American who doesn’t have an Irish, Scottish, German ...or Scottish by way of Ireland, or French, or (name a nationality)... ancestor. The “standard” American accent is supposedly a Midwestern accent, but it’s “standard” in name only. Your accent has everything to do with where in the US you came from, who your parents were, and how much TV you watch.
Because of my accent, or lack thereof, and where I live (DFW), people have always asked me where I was raised. When I ask why, it’s because they think I’m from Ohio. Incidentally, I was born and raised in Arlington, Texas. I have honestly never been to Ohio. However, my paternal grandfather was born and raised in Pennsylvania. He and his heritage is the only Northern, or Yankee, heritage I have. Everyone else, for at least 7 generations is American and from the South.
Maybe I’m rare, but on all sides of my tree, including my paternal grandfather’s line, the immigrant ancestor came to America before The Revolution (and in some cases, long beforehand).