How about Adele, with all her Grammy awards? She sounds thoroughly American when she sings. Then she speaks, and it's working-class London. I was born and raised in Southern California; but one side of the family was from Manchester, UK, and the other a combination of Pacific Northwest and Kansas. In my own speech, I hear an amalgam of all of those (my "u's" are a bit ambiguous.)

Even in California, there is and was a range between "surfer" dialect on one hand and a western, cowboy "twang" on the other. Depends on whether you're raised "by the mountains or the sea."