England was briefly united under Athelstan, thus giving him a claim to recognition as the first King of England. But the Vikings seized control of Northumbria after his death in 939, and it wasn't regained until 954.
Athelstan was succeeded in turn by his half–brothers Edmund I (939–46) and Eadred (946–55), and then by Edmund's son Eadwig ('the Fair'). Edgar, who was Eadwig's half–brother and Æthelstan's nephew, was still a teenager when he came to the throne following Eadwig's mysterious death. As king, Edgar further consolidated the political unity achieved by his predecessors, and his reign became noted for its relative stability. England has remained united ever since, so Edgar is generally recognised as the first King of All England – even though he had himself been responsible for disuniting it when he wrested control of Northumbria and Mercia from Eadwig!
The pinnacle of Edgar's reign was his coronation at Bath in 973, after which he marched his army to Chester. There he was met by six kings of Britain, including the King of Scots, the King of Strathclyde and various princes of Wales. They are said to have signalled their allegiance to Edgar by rowing him in his state barge across the River Dee.
© Haydn Thompson 2017–21