Although in the past the style of British Emperor has been (retroactively) applied to a few mythical and historical rulers of Great Britain, Ireland or the United Kingdom, it is sometimes used as a colloquialism to designate either Plantagenet and Tudor caesaropapism or, more frequently, the British sovereign of the Empire of India.
Mythical British kings
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. The work is a praise-poem and elegy for the 6th-century king Geraint, provides the earliest known reference to Arthur as "emperor".
Britannic Empire
The Britannic Empire was a short-lived breakaway state of the Roman Empire in the late Roman Period. It was formed as a result of the revolt by the naval commander Carausius. It ended when Carausius's usurper, Allectus, was defeated by the Emperor Constantius I in 296.
Imperial ambitions (930–1066) Several Kings of England displayed imperial ambition in the period from 930 to 1066, using a variety of what would be considered imperial titles. The most common title was basileus, but imperator, princeps, augustus, and caesar were all used sporadically. Athelstan (c. 895–939), a collector of imperial relics, had himself proclaimed "imperator" in 930, six years after the death of the last Carolingian emperor. Variants that he used throughout his reign include such formulations as "king of the Anglo-Saxons and emperor of the Northumbrians" (Rex Angulsexna and Norþhymbra imperator). He was also one of many to use the Anglo-Saxon title of Bretwalda ("wielder of the strength of Britain," sometimes rendered Brutenwealde). Edgar the Peaceful (c. 943–975) legendarily took homage from eight lesser kings during an excursion on the River Dee. During his lifetime, he issued royal charters proclaiming himself as Anglorum Basileus and totius Albionis Basileus, among other variants. Ethelred the Unready (c. 966–1016) was noted for being especially fond of high titles during his reign. Canute the Great (994/995–1035) ruled England, Norway, and Denmark through personal union and lacked formal imperial title during his lifetime, though he followed Anglo-Saxon tradition. The lands which he ruled are commonly referred to as the North Sea Empire. Edward the Confessor (c. 1003–1066) continued the occasional use of imperial titles after the restoration of the House of Wessex, but the kings of England largely ceased referring to themselves as emperors following the Norman conquest.
Norman Era: Empress Maud (Matilda)
In this case the epithet "Empress" was rather used to distinguish this person from other princesses called Matilda or Maud. Matilda was not Empress of Britain; she took her title from her previous marriage to Holy Roman Emperor Henry V.
The "Imperium Maius" issue
Although several English monarchs flirted with the idea of "imperial" power, this never led to an official change of the title of "King/Queen" to that of "Emperor/Empress".
Imperium maius
In Christian Europe the use of the title emperor was more than an affectation. A king recognises that the church is an equal or superior in the religious sphere; emperors do not. 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. The distinction began to blur when kings began to claim divine rights.
English kings and the imperium maius
William II, Duke of Normandy (who became William the Conqueror) thought it important enough to request and get a papal blessing for his conquest of England. 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". After Henry I agreed to the Concordat of London in 1107, the English kings recognised the supremacy of the Pope in matters spiritual. For example, when Thomas Becket was murdered, Henry II was forced to recognise that, although he ruled temporal matters, spiritual matters came under the authority of the Church in Rome.
This changed with the dispute between Henry VIII and Pope Clement VII over Henry's wish to have his marriage to Catherine of Aragon annulled. The Act in Restraint of Appeals (1533) explicitly stated that
The next year the First Act of Supremacy (1534) explicitly tied the head of church to the imperial crown: