The monarchy of the United Kingdom, commonly referred to as the British monarchy, is the constitutional monarchy of the United Kingdom and its overseas territories. The monarch's title is "King" (male) or "Queen" (female). The current monarch, Queen Elizabeth II, ascended the throne on the death of her father, King George VI, on 6 February 1952.
The monarch and his or her immediate family undertake various official, ceremonial, diplomatic and representational duties. As the monarchy is constitutional, the monarch is limited to non-partisan functions such as bestowing honours and appointing the Prime Minister. The monarch is, by tradition, commander-in-chief of the British Armed Forces. Though the ultimate formal executive authority over the government of the United Kingdom is still by and through the monarch's royal prerogative, these powers may only be used according to laws enacted in Parliament and, in practice, within the constraints of convention and precedent.
The British monarchy traces its origins from the petty kingdoms of early medieval Scotland and Anglo-Saxon England, which consolidated into the kingdoms of England and Scotland by the 10th century AD. In 1066, the last crowned Anglo-Saxon monarch, Harold II, was defeated and killed during the Norman conquest of England and the English monarchy passed to the Normans' victorious leader, William the Conqueror, and his descendents.
In the 13th century, Wales, as a principality, became a client state of the English kingdom, while the Magna Carta began a process of reducing the English monarch's political powers.
From 1603, when the Scottish monarch King James VI inherited the English throne as James I, both the English and Scottish kingdoms were ruled by a single sovereign. From 1649 to 1660, the tradition of monarchy was broken by the republican Commonwealth of England, which followed the War of the Three Kingdoms. The Act of Settlement 1701, which is still in force, excluded Roman Catholics, or those who marry Catholics, from succession to the English throne. In 1707, the kingdoms of England and Scotland were merged to create the Kingdom of Great Britain, and in 1801, the Kingdom of Ireland joined to create the United Kingdom of Great Britain and Ireland. The British monarch became nominal head of the vast British Empire, which covered a quarter of the world's surface at its greatest extent in 1921.
In the 1920s, five-sixths of Ireland seceded from the Union as the Irish Free State, and the Balfour Declaration recognised the evolution of the dominions of the empire into separate, self-governing countries within a Commonwealth of Nations. After the Second World War, the vast majority of British colonies and territories became independent, effectively bringing the empire to an end. George VI and his successor, Elizabeth II, adopted the title Head of the Commonwealth as a symbol of the free association of its independent member states.
The United Kingdom and fifteen other Commonwealth monarchies that share the same person as their monarch are called Commonwealth realms. The terms British monarchy and British monarch are frequently still employed in reference to the shared individual and institution; however, each country is sovereign and independent of the others, and the monarch has a different, specific, and official national title and style for each realm.
Sources : Wikipedia