Who were the Britons, and what are the four nations of Britain?

The Britons (meaning those who live in Britain) were one of few native peoples inhabiting Britain before the Anglo-Saxon invasions of the fifth century; they were once believed to be descendants of the Celts. The four nations that comprise the modern-day United Kingdom of Great Britain and Northern Ireland are Wales, which was annexed by the Laws in Wales Act of 1542 by King Henry VIII; Scotland, following its union with Great Britain with the 1707 Acts of Union; Northern Ireland, after Ireland joined in union with the United Kingdom in 1801 and five-sixths of Ireland seceded in 1922, with Northern Ireland being the sixth piece to stay; and England. As of 1922, England, Wales, Scotland, and Northern Ireland are the four nations that comprise the United Kingdom.
https://learnenglish.britishcouncil.org/general-english/word-on-the-street/loch-ness/four-nations