NPR had a hollywood dialogue coach on some time ago talking about how your "classic" British accent (if there is such a thing) is almost identical to the old antebellum Southern accent. the only difference was that the Southern accent was more tonal. That might be a reason why so many Southern characters in the movies and teevee are played by British actors. Think Vivian Leigh in Gone With the Wind, Andrew Lincoln in Walking Dead, Stephen Moyer in True Blood, and every single character with a Southern accent in Blackhawk Down.
Additionally, your more Appalachian accents tend to be more Scottish...kinda clipped and nasally (Liam Neeson in Next of Kin).
Then when you get more towards Louisiana, it's a mix of all kinds of accents. Have you ever noticed how some people from around New Orleans sound almost like they have New Jersey accents? Was there a lot of French influence up in Jersey?.