South Britain is a term which was occasionally used in the 17th and 18th centuries, for England and Wales

England and Wales

England and Wales is a jurisdiction within the United Kingdom. It consists of England and Wales, two of the four countries of the United Kingdom...

 in relation to their position in the southern half of the island of Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

. It was used mainly by Scottish

Scottish people

The Scottish people , or Scots, are a nation and ethnic group native to Scotland. Historically they emerged from an amalgamation of the Picts and Gaels, incorporating neighbouring Britons to the south as well as invading Germanic peoples such as the Anglo-Saxons and the Norse.In modern use,...

 writers, in apposition to the term "North Britain

North Britain

North Britain is a term which has been occasionally used, particularly in the 17th and 18th centuries, for either the northern part of Great Britain or to Scotland, which occupies the northernmost third of the island...

", which generally referred to Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

.

Origins

Early uses of the designation have been noted after the 1603 Union of the Crowns

Union of the Crowns

The Union of the Crowns was the accession of James VI, King of Scots, to the throne of England, and the consequential unification of Scotland and England under one monarch. The Union of Crowns followed the death of James' unmarried and childless first cousin twice removed, Queen Elizabeth I of...

 of the Kingdoms of England

Kingdom of England

The Kingdom of England was, from 927 to 1707, a sovereign state to the northwest of continental Europe. At its height, the Kingdom of England spanned the southern two-thirds of the island of Great Britain and several smaller outlying islands; what today comprises the legal jurisdiction of England...

 and Scotland

Kingdom of Scotland

The Kingdom of Scotland was a Sovereign state in North-West Europe that existed from 843 until 1707. It occupied the northern third of the island of Great Britain and shared a land border to the south with the Kingdom of England...

. King James VI & I

James I of England

James VI and I was King of Scots as James VI from 24 July 1567 and King of England and Ireland as James I from the union of the English and Scottish crowns on 24 March 1603...

 used the terms "South Britain" and "North Britain" for England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 (and, implicitly, Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

) and Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 respectively, most famously in his Proclamation of 1606 (here) establishing the first Union Flag

Union Flag

The Union Flag, also known as the Union Jack, is the flag of the United Kingdom. It retains an official or semi-official status in some Commonwealth Realms; for example, it is known as the Royal Union Flag in Canada. It is also used as an official flag in some of the smaller British overseas...

, where Scotland and England are not otherwise named:
This usage was repeated in Charles I

Charles I of England

Charles I was King of England, King of Scotland, and King of Ireland from 27 March 1625 until his execution in 1649. Charles engaged in a struggle for power with the Parliament of England, attempting to obtain royal revenue whilst Parliament sought to curb his Royal prerogative which Charles...

's Proclamation of 1634 on the use of the flag, though adding England and Scotland too for explanation: