English only in southern England? mmmmm.
ok take Britannia Ruled,
the Occupation makes England much more nationalistic in the short run so Anne not James becomes Queen of England, England is poorer because of the occupation and puritans are died or living in Scotland so no colonies.
thanks to the occupation of England and Holland by Spain Scotland became the hub of calvinism. because of that and the Stewarts messing up, Scotland becomes a Commonwealth like Cromwell's,
the Church of England become very "high church" after the occupation and many people (Especially in the North of England) feel that it may as well be Catholic.
so in 1680's Scotland invades northern England and takes over, southern England (with French help) holds out.
Scotland being nationalistic speaks Scottish Gaelic, northern England speaks a Gaelic English mix known as Northumbrian.
southern England speaks mostly what we'd know as English, with more French & Gaelic loan words