North Britain - Origin

Origin

Early uses of the designation have been noted after the 1603 Union of the Crowns of the Kingdoms of England and Scotland. King James VI & I used the terms "South Britain" and "North Britain" for England and Scotland respectively, most famously in his Proclamation of 1606 (here) establishing the first Union Flag, where Scotland and England are not otherwise named:

"Whereas some difference has a risen between our Subjects of South and North Britain, Travelling by Sea, about the bearing of their flags"

This usage was repeated in Charles I's Proclamation of 1634 on the use of the flag, though adding England and Scotland too for explanation:

"Our further will and pleasure is, that all the other Ships of Our Subjects of England or South Britain bearing flags shall from henceforth carry the Red Cross, commonly called S. George’s Cross, as of old time hath been used; And also that all other ships of Our Subjects of Scotland or North Britain shall henceforth carry the White Cross commonly called S. Andrews Cross"

After the Act of Union 1707, Scotland was sometimes referred to as "North Britain". In 1707, the Royal Scots Greys were renamed the "Royal North British Dragoons". In Rob Roy (1817), Sir Walter Scott refers to a Scottish person in England as a North Briton, sometimes in the mouth of an English character but also in the authorial voice.

"Why, a Scotch sort of a gentleman, as I said before," returned mine host; "they are all gentle, ye mun know, though they ha' narra shirt to back; but this is a decentish hallion—a canny North Briton as e'er cross'd Berwick Bridge — I trow he's a dealer in cattle." —Scott, Rob Roy