We know frighteningly little about the process of the Anglo-Saxon conquest, the battle of Dyrham was important because it almost certainly led to the integration of British Hwicce into Mercia and broke the power of the urban centres of the region around Gloucestershire, Somerset and the Chilterns.
I would actually put the key date for near irreversible Anglo-Saxon cultural dominance of all of today's England at 617, and the conquest of British Elmet around Leeds and the Pennines. Having said that, Cadwallon of Gwynedd's conquest of Northumbria in 634 could have been enough to create a strong Northern polity which could have led to a British reconquest of south central England, or an eventual partition of Britain into 4 kingdoms rather than 3. There were clear British elements in Anglo-Saxon elites in Mercia at this time, and there would be in Wessex until the 8th century. The population of Anglo-Saxon England (outside the South-East) must have contained British-speaking elements up to the Viking era.
Of course we don't know enough about how strong or big Elmet was, or what Cadwallon's plan was, to be sure of this.