That raises another critical point: The "Southern" accent is itself subdivided into several others, including but not limited to the Piedmont, Appalachian, Deep South and Texas varieties. The Texas twang is especially distinctive, best approximated as a mix of Southern drawl and the nasal Midwestern accent, and found nowhere else (to my knowledge) except Oklahoma (i.e. northern North Texas.)

A large part of QUEBEC speaks French; most of Canada speaks English with its own accent, closest to Midwestern or New England ones (the latter at least as close to RP as Virginias Piedmont accent) in the same way multiple Southern accents are closer to each other than to ANY other. Once we start considering RPs proximity to both Green Mountain and Piedmont accents which geography widely separates from each other, then the closely related English accents found in Canada and half of Africa, any claim Southern accentS have to be the most common English one becomes dubious.