While non-rhotic accents are common in parts of America, the dominant theory among linguists is that this was a result of Americans copying a trend in England, not the other way around, as you seem to be asserting:
> By the 1770s, postvocalic /r/-less pronunciation was becoming common around London even in formal educated speech. The English actor and linguist John Walker used the spelling ar to indicate the long vowel of aunt in his 1775 rhyming dictionary.[4] In his influential Critical Pronouncing Dictionary and Expositor of the English Language (1791), Walker reported, with a strong tone of disapproval, that "the r in lard, bard,... is pronounced so much in the throat as to be little more than the middle or Italian a, lengthened into baa, baad...."[8] Americans returning to England after the American Revolutionary War, which lasted from 1775 to 1783, reported surprise at the significant changes in the fashionable pronunciation that had taken place.[16]
...
> The loss of postvocalic /r/ in the British prestige standard in the late 18th and the early 19th centuries influenced the American port cities with close connections to Britain, which caused upper-class pronunciation to become non-rhotic in many Eastern and Southern port cities such as New York City, Boston, Alexandria, Charleston, and Savannah.[9] Like regional dialects in England, however, the accents of other areas in the United States remained rhotic in a display of linguistic "lag", which preserved the original pronunciation of /r/.[9]
It's interesting the way that words sometimes seem to have nuance that doesn't seem to be strictly within the definition.
For no reason I can remotely explain, I'd divide these into clever/bright and smart/gifted.
I've met several autists that I would readily describe as smart or even gifted, but I wouldn't describe as clever or bright.
(Although the distinction might just be that I'm an arsehole.)