It's fairly simply according to wiki...
Traditionally, the Kingdom of England is usually considered to begin with Alfred the Great, King of Wessex, one of the petty kingdoms to rule the southeastern portion of Great Britain. While Alfred was not the first king to lay claim to rule all of the English, his rule represents the first unbroken line of Kings to rule the whole of England, the House of [...] By the late ninth century Wessex was the dominant Anglo-Saxon kingdom. Its king, Alfred the Great, was overlord of western Mercia and used the title King of the Angles and Saxons, but
he never ruled eastern and northern England, which was then the Danelaw. His son Edward the Elder conquered the eastern Danelaw, but Edward's son Æthelstan became the first king to rule the whole of England when he conquered Northumbria in 927, and he is regarded by some modern historians as the first king of England...
However, England came under the control of Sweyn Forkbeard, a Danish king, after an invasion in 1013, during which Æthelred abandoned the throne and went into exile in Normandy. English rule then shuffled back and forth between the House of Wessex and subsequent Danish invaders until William I who secured the kingdom against further invasion and subdued the warring Earls as never before.
https://en.wikipedia.org/wiki/List_of_English_monarchs