The term "emperor" has sometimes informally been retroactively applied to a few mythical and historical rulers of Great Britain, Ireland or the United Kingdom.
The mythical British ruler King Arthur is referred to in medieval Welsh texts as ameraudur (meaning "emperor").
The Welsh poem Geraint, son of Erbin, written in the 10th or 11th century, describes a battle at a port-settlement and mentions Arthur in passing.
This was illustrated by Henry VIII of England who started to use the word imperium in his dispute with Pope Clement VII over the annulment of his first marriage.
Richard I refused to show deference to Henry VI, Holy Roman Emperor when held prisoner by him, declaring "I am born of a rank which recognizes no superior but God".
[7] After Henry I agreed to the Concordat of London in 1107, the English kings recognised the supremacy of the Pope in matters spiritual.
The Act in Restraint of Appeals (1533) explicitly stated that Where by divers sundry old authentic histories and chronicles it is manifestly declared and expressed that this realm of England is an empire, and so hath been accepted in the world, governed by one supreme head and king, having the dignity and royal estate of the imperial crown of the same.
"[11] The British government led by Benjamin Disraeli conferred the additional title Empress of India on Queen Victoria by an Act of Parliament,[12] with effect from 1 May 1876, reputedly to assuage the monarch's irritation at being, as queen, notionally inferior to her daughter Victoria, the future German empress.