BBC has an article on dialect change. Final /r/, which has been absent from The King’s English for many centuries, persisted in isolated areas. Now it’s disappearing in the isolated areas as well.
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Accent change is often like a puddle: it dries up in most places and leaves remnants around the edges, hence why Cornwall and East Lancs behave similarly today. The study found the strongest /r/ was spoken by older men, and was more prominent during formal speaking, which raised “interesting questions about social prestige and clearness of speech”.
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Well, the prestige questions are fairly obvious, but the new change itself is interesting.
In US the ‘received pronunciation’ has changed in both directions at various times. The newer parts of the country have strong final /r/, for reasons that aren’t clear. Possibly from Scotch-Irish origins instead of London.
The northeast, where London influence was pure, retained the London weak /r/.
In the age of movies and radio, a weak /r/ was official. It seems to have come from Upstate New York, without some of the peculiarities of NYC.
Television’s top influencers were Midwestern, not NYC. Uncle Walter was born in St Joseph. Johnny Carson was Omaha. Dave Letterman was Indianapolis.
The television-era strong /r/ is still dominant in the newer media. Quick example is this interview between Saagar and Tucker, from very different origins. (Tamil and WASP.) Both are speaking the same dialect I grew up with in Kansas.
In short the two countries are moving farther apart, not converging as the usual theories of global communication would assume.
Polistra's Mill 