British Empress

The British Empress (Emperor if the sovereign is male) is the sovereign of the British Empire, serving as the monarch of the United Kingdom, its Dominions and dependencies, and Lord Protector of the British Protectorates.

History
Following the death of Henry, Duke of Cornwall, in 1504, England faced a succession crisis, as Henry VII lacked a male heir. The King put some thought into remarriage, specifically to Catherine of Aragon, the widow of his eldest son, Prince Arthur - however a male heir born of the marriage would likely be underage at the time of his father's death, requiring a regency, which had a long record of causing instability in England. Henry VII was thus forced to turn to his daughters - the eldest, Margaret, was married to James IV of Scotland, so her ascension would result in a union of the crowns. This would remove a thorn from England's side, though some nobles disliked the idea of a foreign king, instead advocating the King's younger daughter Mary - many likely having the idea to marry her and become king themselves. With James IV having declared that he'd defend his wife's and son's rights to the English throne, Henry confirmed Margaret as his heir.

House of Windsor (1901 - present)

 * The heir apparent is The Queen's eldest son, Charles, The Prince of Wales. The heir apparent's heir is his eldest son, William, The Duke of Edinburgh; his heir is his eldest son, Prince George of Edinburgh.