Alfred continued the practise of the past by issuing joint coinage with both Mercian kings, Burgred and Ceolwulf (874-879), who had been put in place by the Danes after Burgred had been forced from Mercia. By the mid 870’s some coins even showed Alfred as ‘king of the English’.

The control of London and east Mercia by the Danes was a threat to both the West Saxons and the Mercians. In 886 London was besieged and captured by Alfred. He gave the control of London to Æthelred, Lord of Mercia, who had acknowledged Alfred’s position of over-lordship in 883. 

The defeat of the Danes in London was hailed by both the Anglo-Saxon Chronicle and Asser as a great victory, with both proclaiming that now London had fallen Alfred was now overlord of all Saxons outside of Viking control.

Treaties signed with Guthrum, the Viking King of East Anglia and charters issued at the time give weight to the fact that Alfred was seen as the ‘king of the Anglo-Saxons’ at the time.

That being said Alfred's influence over England as a whole was limited, 'Danelaw' ruled the other side of Watling Street, a Roman road that ran from Shropshire to Kent. On the other side of the Watling Street there was a mixing of Viking and Saxon culture, however those in power were the Vikings, and their customs took precedence.

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a6/England_878.svg/512px-England_878.svg.png
Map showing the land held by the Danes and English in 878
Hel-hama, England 878, CC BY-SA 3.0