The British monarchy was a hereditary line of men and women in the United Kingdom on Earth who served as the heads of state of that nation and its territories. The modern line could be traced in an unbroken line back to the Norman leader William the Conqueror in 1066, and over the centuries these sovereigns played a major role in Earth history.
John of England was an early British monarch who lived during the late 12th and early 13th centuries. A series of disagreements with Pope Innocent III and various British nobility led to John being forced to sign the Magna Carta, a key document in human history which established the rights of the governed. (TOS episode: "Court Martial")
Henry V (1387–1422) was sovereign from 1413 to 1422. He led England to a number of important military victories, and later became the subject of a celebrated play by William Shakespeare. Lt. Commander Data once played Henry in a holodeck program, in a scene in which the king disguised himself as a common soldier, as part of his efforts to better understand humanity. (TNG episode: "The Defector")
In the 20th and early 21st centuries, the monarchy was headed by Queen Elizabeth II. Taking the throne upon the death of her father George VI on February 6, 1952, she reigned as monarch for over 60 years. In 1994, she and French President François Mitterrand attended a gala event in Calais to mark the official opening of the Channel Tunnel. (TOS novel: The Rise and Fall of Khan Noonien Singh, Volume 2 and historical accounts)
The monarchy survived into at least the mid 21st century. After the reign of Elizabeth II, the monarchy was eventually headed by Queen Mary. After the Optimum Movement came to power, she was captured by their forces and imprisoned in London. Other members of the British royal family went into hiding or exile. (ST novel: Federation)