In the fifth century the kings of Dumnonia expanded their territory east and north to encompass all of southern England, including OTL London. They are able to resist Anglo-Saxon invaders on the south coast, though there are Anglo-Saxon settlements on the east coast and in OTL East Anglia.
In the following centuries British kingdoms in OTL England and Wales such as Dumnonia, Rheged and Gwynedd compete for power. They manage to confine the Anglo-Saxon petty kingdoms to eastern England. By the year 800 these kingdoms are subordinate to Dumnonia, which now is overlord of other kingdoms south of the Forth-Clyde line in OTL central Scotland.
Viking raids and invasions happen as in OTL. But Dumnonia is able to resist Viking conquest.