Kalu
Great Britain/Britain is the official collective name of England, Scotland and Wales. This does not include Northern Ireland, which is part of the UK.
FNOUK? Only sung by those school children who have not yet broken up for end of term??
Feeling pedantic today. ?

I was, too.
The British Isles:
British Isles, group of islands off the northwestern coast of Europe. The group consists of two main islands, Great Britain and Ireland, and numerous smaller islands and island groups, including the Hebrides, the Shetland Islands, the Orkney Islands, the Isles of Scilly, and the Isle of Man. Some also include the Channel Islands in this grouping.
Great Britain:
Great Britain is an island separated from the European mainland by the English Channel and North Sea. It comprises the nations of England, Scotland and Wales.
United Kingdom:
United Kingdom, island country located off the northwestern coast of mainland Europe. The United Kingdom comprises the whole of the island of Great Britain—which contains England, Wales, and Scotland—as well as the northern portion of the island of Ireland. The name Britain is sometimes used to refer to the United Kingdom as a whole.
Britannica
Britain might seem an appropriate term though, as occasionally we don't seem very united.
Which brings us back to the song.