It's actually more likely to be a holdover from very old British English influence - the Irish and Scots press-ganged white slaves who fled into the hills of Virginia and became the rednecks of coal country. I know that's where I get most of my dialect, even though I grew up in Indiana (so I also get a Blues-Brothersish Chicagoan avoidance of theta, to end up with that unique Hoosier accent that non-obsessives don't notice).

London doesn't have a monopoly on glottal stops.