sorry my bad. actually i wanted to write Australia, Britian and Canada or simply as ABC but i guess i must have been dreaming........trust me i am good at geography and i did took regional, physical and world geography class too. 
nice catch by  jhapaliketo.

Anyway technically speaking, The United Kingdom of Great Britain is the officaial name, also commonly known as the United Kingdom, the UK, or Britain
It constitutes England(englishmen), Scotland(scottish) to the north, Wales(Welsh) to the west and Ireland(Irish)to the north west.

When combine, the people are simply called the British.

The Kingdom of Great Britain was created by the political union of the Kingdom of England (which included Wales) and the Kingdom of Scotland. This event was the result of the Treaty of Union that was agreed on 22 July 1706.
Almost a century later, the Kingdom of Ireland, already under English control by 1691, merged with the Kingdom of Great Britain to form the United Kingdom.
Hence after merging of see each Kingdom(England + Scotland + Wales + Ireland) = United Kingdom
That's how the Union Jack(flag) came to be.  England flag symbol is red cross in white background
Scotland flag is white cross in blue background.

When combine, it becomes a Union Jack

Hope this help.