Southern England, properly speaking, is that part of England which lies between the English Channel and the River Trent. It was the portion allotted to Henry I after his armies had been defeated in the Battle of Newark by the forces of John Uskglass, who divided England between the Norman kings and himself.
A peculiar constitutional irregularity was created however when the Raven King withdrew from his realm of Northern England in the fifteenth century. Rather than leave the lands which lay between itself and Scotland to lapse into unruly barbarism, and wary of annexing them outright, the king of Southern England and his council undertook to rule them in the name of their absent lord. Thus the two halves of England, divided since the conquest of 1111, became once more a united kingdom under the King of England and his Parliament at Westminster. Technically however the two remain potentially separate - depending on whether John Uskglass should ever decide to return and resume his royal duties.