It was not formed by any union. Great Britain refers to the island containing England, Wales and Scotland. You may be confusing it with the United Kingdom, which consists of Great Britain and also Northern Ireland, which is on the neighbouring island of Ireland.
England and Scotland
Baptist Union of Great Britain was created in 1813.
Karate Union of Great Britain was created in 1966.
Union of German Socialist Organisations in Great Britain was created in 1941.
The Parliament of Great Britain was created in 1707 through the Act of Union between England and Scotland. This union merged the separate Parliaments of England and Scotland into one, creating the Parliament of Great Britain.
England and Scotland
The union of the Parliaments of England and Scotland in 1707.
The US, Soviet Union, and Great Britain
Miss Great Britain was created in 1945.
Great Britain Ltd was created in 1982.
Scotland and England created a union called Great Britain in 1707. England did not conquer Scotland.
By the 1707 Act of Union
Air Defence of Great Britain was created in 1943.