The answer to this question depends on how you define “King of England.” After the Roman Empire's decline, several Saxon clansmen and “kings,” as well as Scandinavian invaders ruled different regions of England and Britain. The kings who eventually evolved into the kings of the whole of England, were the kings of Wessex and were crowned by the Archbishop of Canterbury.
The First Kings in England
Both Egbert, king of Wessex and Offa, king of Mercia are sometimes called the first kings of England. Offa dominated a large part of southern England in the late eight century, but his descendants did not manage to keep the area as a kingdom. Egbert, king of Wessex, managed to conquer Mercia in 829, but he, too, lost control over this territory. Wessex was the largest Anglo-Saxon kingdom by the late ninth century and Alfred the Great was crowned as “King of the Angles and Saxons.” He ruled over western Mercia, but not rule northern or eastern England (Danelaw).
First King of The Whole Of England
After Edward de Elder conquered eastern England (Danelaw), Athelstan had most of England under his control. He added Northumbria to his kingdom, which made him the first king of all England.
First “King” of the United Kingdom of Great Britain
The first monarch that ruled Great Britain was a queen and not a king. Queen Anne was crowned as Queen of England, Ireland and Scotland in 1702 and in 1707 two of the kingdoms, England and Scotland became one state: the United Kingdom of Great Britain.