A rough guide is shown in the map. Alfred performed well (from a nadir of Island in the Marshes of Alderney - from memory) to stalemating the Danes at the line shown on the map - about 50% of current day England.
His son Edgar pushed the line back to the Humber and Danes South of there submitted to him. However North of the Humber was a mess, with the Vikings in York, a separate kingdom of Starthclyde and the rump of the Old Saxon Kingdom of Northumbria and the Scots all slaughtering each other.
As you can see Athelstan completed the unification of England, hammered the Scots (as usual) and was acknowledged as an overlord (whatever that meant) by the Welsh.
England would never be as large again. And although, foreigners in the form of Sveyn and Knut, Hathaknut, Harold Harefoot would rule the centralised kingdom survived - passing to Edward the Confessor and Harold Godwinson.
William the Norman, was unable to hold the hold the North and burned and destroyed it to prevent a successful Danish invasion. He permanently lost the Lothian region (ie from the current English border to Edwin's Burgh (Edinburgh)).
So much for the mighty Norman - also lost was the overlordship of Wales which slipped from William's grasp.