That indeed explains what you are meaning if by "the British accent" you are actually meaning RP - but no I did not know that was what you were meaning. That is again only one form of accent in Britain. And of course most folks in Britain don't speak RP and I imagine it was even more so for the progenitor of RP in say the 18th or 17thC or so. Wasn't it a form of speech in an area of southern England?Certainly for me here in the Scottish Borders if you read the dialogue in say a book like James Hogg's "The Three Perils Of Men" then the accents are very obviously pretty close to how locals in the Scottish Borders speak now today. The same for earlier written pieces that survive in the likes of Border Papers etc concerning legal matters, Warden's correspondence etc. Folks from southern and other parts of Scotland then went to Ulster and that is clear in the accents of Ulster Scots speakers now. Billy Kay suggests Ulster Scots is closest to Ayrshire Scots which makes sense as it is close to that area and many of the folks who moved went from south-west Scotland. From Ulster they went further afield and aren't Ulster Scots (or as they are called in the US the Scotch Irish) particularly associated also with the Appalachian region you mention? There probably is indeed remnants of their ancestor's Scottish speech in their current dialect but it is nowhere near as close to the way folks in the 17thC Borders(for instance) would have spoken as the speech of modern Borderers is. Likewise for the other dialect/accent areas. I imagine many people from any of the major English speaking countries might speak as close or closer to historic RP than many folk in Britain do. Simply because accents here are so diverse!