"Great Britain, or Grande Bretagne, was, I believe, originally used to distinguish the island of Britain from Brittany."

It goes back a lot further - the Romans used Brittania Major to differentiate it from the other British Islands, notably Brittania Mino, otherwise known as Ireland.

After 1707, the Scots put a lot of effort into the concept of "Britain" - look at all the variants of North Briton in the late 18th and early 19th century, but getting no response from our Southern neighbours who appeared to think that everybody was now English, went back to Scottish Nationalism.