My earliest memory of language was speaking in an English dialect - I was born in the States but moved to Britain at 3, lived in a small village in Norfolk, spent my preschool days watching Television for Schools on BBC and ITV, etc. When I moved Stateside as a first-grader I had the worst time - first, we were living in Florida where the accents were thick as molasses (think Fort Walton Beach in the late 70s before it became a touristy place). I was always being asked to slow down. Americans in the South speak incredibly slowly, as a general rule.
Back to the UK from the age of 9 to 15, I found that while I didn’t accrue much of an accent (mostly a conscious thing - it was such a strong part of your identity as a Yank in England to maintain your accent, and the kids who sounded English were mocked in my school), I certainly kept the cadence, and their are accents in southern Britain that aren’t a million miles from the mid-Atlantic, midwestern American accent. I never once had difficulty understanding Brits for the most part…
…except Geordies and Scousers and Mancs. Really thick Northern accents are hard for me to follow. What’s funny, though, is that when Oasis got big here in the mid-90’s they would have subtitles on when Noel Gallagher gave interviews. I thought this was extremely lazy on the part of the broadcasters, who apparently thought we Americans were so thick and slow we couldn’t understand a guy who didn’t speak Estuary English. Noel’s quite easy to understand; maybe his brother Liam is a little harder to understand, partly because I don’t think he’s all that bright.
I watch EastEnders (soap opera that airs on the BBC) and most of the characters are quite easy to understand, though again, the cadence is quick. I’m used to it, but my wife has a hard time following the dialogue. There are a few characters that are almost completely incomprehensible: two characters with thick Manc accents, and one character with a strong Cockney accent that talks fast and low (Max Branning, for anyone who’s familiar).
I certainly watched tons of American TV as a kid in the mid 70s and early 80s - we had all the popular shows from the US, so the accents are quite familiar to most Brits I imagine. I do think that most actors playing Americans or using American accents weren’t that convincing. Hugh Laurie is quite skilled with his House accent, even though it’s a very generic American accent - but it works. I find most fake American accents by British actors to be too nasal.
Speaking of which, I’m always amazed by how American some of my favorite Scottish artists sound. Gerry Rafferty, Jim Kerr, Midge Ure… sound so different when they speak compared to how they sound when they sing.