Or at least a version of Brittonic Celtic, the language that was spoken, along with Latin, when the Romans left Britain in the early 400s. Within a relatively short time the whole of what became England, or at least its southern part, was speaking English. We know this because the number of Celtic placenames in southern England is insignificant, replaced almost entirely by English names.
Originally it was assumed this shift resulted from mass immigration and population replacement. Then archaeologists assured us that the number of settlers was very low. But if this were so we might have expected the native language to survive rather than disappear outside Wales and Cornwall. If population replacement was not the reason it must have resulted from a small elite imposing their language on the mass of the native population. This is supposed to have occurred in the main peacefully, despite the frequent references in the Anglo-Saxon Chronicle to warfare between English and Britons. English archaeologists, embarrassed by the possibility that their ancestors may have indulged in ethnic cleansing, were keen to embrace this model.
It was never accepted by placename specialists, who pointed out that the later elite takeover by the Norman-French had very little effect on English placenames. We could also look further afield. The elite British dominance of India after the 1760s didn’t lead to the disappearance of native languages either.
More recently, DNA evidence suggests that the English immigration could have been on a larger scale, amounting to around ten per cent of the native population. Given a reproductive advantage, this proportion could have imposed their DNA within 300 to 450 years, which is coincidentally very similar to the period of time over which the language shift took place.
It looks as if the English settlement of Britain was more like the European takeover of North America. Of course, this didn’t just involve warfare, conflict and the ethnic cleansing of the native Americans, but trading, cooperation at times and intermarriage. Nevertheless, it resulted in a similarly comprehensive takeover and the extermination of several native languages.
The English cultural takeover of Britain came to a shuddering halt when they reached the Tamar, when they appear to have been content to leave the natives using their own tongue in their reservation beyond the river. The second question therefore becomes why didn’t the Cornish of the time speak English (like the Britons in Devon and further east)? Wholesale language replacement ground to a halt in the ninth and tenth centuries and Cornish survived west of the Tamar for another 800 years or so.
Were we just lucky? Had the English lost their enthusiasm for linguistic colonisation by the time they reached the Tamar? Or were the Cornish in those centuries in a position to trade political submission for cultural autonomy? Did their possession of the valuable resource of tin give them a bargaining chip? Furthermore, this speculation suggests that the answer could be political. Maybe the Cornish were more organised than British communities to the east when the English rode over the horizon.