Just where is this place "southern Scotland"? Do we mean the lowlands? Even that word has different definitions: geographically the lowlands are south of the fault-line, but traditionally hieland and lalland were synonyms for Gaelic- and Scots-speaking, and with the decline of Gaelic the status of, say, Caithness is rather uncertain - and then the northern isles belong properly to neither. Or the Borders? "Southern Scotland" is a term we hardly ever use culturally or geographically. In only comes up as the baldest of descriptive terms: the only thing I can think of called "Southern Scottish" off the top of my head would be military districts.
Galloway could probably end up English or affiliated with England with a medieval PoD.