wodmarach: RP which most people think of as the aristocratic accent is a taught non localised dialect used for ease of understanding.
It's an interesting difference, because the class distinction in language in the US is sort of the opposite.
The taught, non-local dialect (SAE, "Standard American English", also called "newscaster") carries no prestige, other than not interfering with getting a job in broadcasting ("in television you are not supposed to sound like you're
from anywhere" [Linda Ellerbee, from Texas]).
But the prestige "posh" dialects are local and not explicitly taught: Long Island Lockjaw (think Katharine Hepburn); Boston Brahmin (Maj. Winchester on M*A*S*H, though it is not David Ogden Stiers' native dialect); Virginia Tidewater (Vivien Leigh as Scarlett O'Hara).
(British actors can often pull off effective performances as US Southerners, because Southern dialects like Tidewater have diverged less from their British origins.)
And some local dialects vie with some of the British for incomprehensibility: Yat (New Orleans); Picksburgher; Hawaiian pidgin; Chicano.