The English spoken in the Deep South is the American dialect that has the most in common with British English.
It depends on what type of British English we're talking about. The Queen's English? Perhaps, the dialect of the lower South descends largely from migrants from the London region back during the colonial period, along with an additional substantial input from the West Country. But there are a lot of varieties of British English, both today and back before American dialects split from British ones. For example, the dialect of the Upper South has a lot in common with 18th century northern English and lowland Scottish dialects, and the dialect of New England has a lot in common with 17th century East Anglian dialects. It has a lot to do with what type of people migrated.
