This question is really a part of a much bigger debate on exactly how Anglo-Saxon England was formed. Great debate has raged over the extent to which there was a "great migration" of Angles, Saxons and Jutes to England in the sixth century, which displaced much of the Romano-British population, or forcibly integrated them. Procopius, Gildas and Bede's Historia Ecclesiastica all record that the Britains were put to flight by the English, who settled and established themselves as a dominant culture, simply bringing their language with them. A stark change in material culture in the period could be used to suggest that this was the case.

However, archaeological and dna evidence suggest this was not so much the case. Settlement archaeology shows no great change in land use during this period, which would have occurred had the Romano-British fled en masse. Instead, cemeteries in the period show a synthesis of Romano-British and Anglo-Saxon cultural styles with the emerging "English" styles eventually proving dominant (Hills and Lucy [2013], Spong Hill IX: Chronolgy and Synthesis). Similarly, recent DNA analysis on broad swathes of the English population shows that only approximately 20-40% of the population is actually Anglo-Saxon in origin, with much higher proportions on the east coast, diminishing further West (Capelli et al. [2003], "A Y chromosome census of the British Isles" Curr. Biol. 13, 979–984).

What all of this implies is that the Anglo-Saxon-ification of England was less of a replacement, and more of a migration of an elite, who placed themselves firmly in charge of the culture and began to assimilate downwards. Linguisitically, Brythonic appears to have had more of an influence on Old English than previously thought. The Germanic language spoken in England in c.700 has changed substantially from that spoken by the first Cerdings who arrived in the 500s, and was remarkably different from continental Saxon, bearing many changes, pronunciations and grammar structures from Brythonic ( Hickey [1995], 'Early Contact and Parallels between English and Celtic', Vienna English Working Papers 4: 87–119). It appears to be the case, therefore, that Old English developed as a hybrid as a native population assimilated and altered the language of their new masters. The change is likely to have been slow and uneven, with areas more densely settled such as the south-east switching quite rapidly to speaking Old English as Anglo-Saxon cultural dominance was rapidly established, while the language took far longer to pass into use in the Western areas. Thomas Toon suggests that for quite some time, areas of central England would have been bilingual, bracketed by predominantly Brythonic-speaking areas to the West and predominantly English-speaking regions to the East. (Toon [1983], The Politics of Early Old English Sound Change).