How old is English?
It is not enough to propose the theory that Britain had two languages before it was conquered by
the Romans. Many events during the 5th century AD are interpreted by most authors accordingly to the idea that the Anglo-Saxons
disturbed the British society so much that they could even impose their language upon the local population. The 5th century was
seemingly a Dark Age for more than one reason. However, once one accepts the idea that this official theory is probably false,
one can and should re-interpret the events of that century. That's what I'm doing in this chapter.
 This is deductive - the idea originates from the assumption that the Welsh liked the Roman way of life. Mentality and language in the east of Britain made that eastern lords maintain a (slightly) different, more northern tradition. This seems to be confirmed by archaeological findings: the villas in the west were more sumptuous.
An excellent contemporary text illustrating the social situation during the 4th century can be read by clicking here (external link - read the section: The German challenge to the Roman Empire).
Late Roman Britain
After the Romans had conquered Britain, they found in the “Welsh” (all western Britons) a natural ally. The attitude of this population towards the Romans must have been very similar to the one in Gaul. Tacitus clearly stated that the Gauls were fond of the Romans and glad to be called Gallo-Roman. The Romans found a vague language border some 30 km to the east of today’s Wales and the Welsh speakers must have formed 20% of the population of Britain. During the Bronze Age the language border was located more or less upon the borders of the hydrographical basins (or watershed) of the rivers flowing into the Atlantic. This Bronze-Age-Wales consisted of these regions: Cumbria, northwest England (greater Cheshire), modern Wales, the valley of the Severn, Salisbury plain and greater Cornwall. Bronze-Age-Wales had been ‘under pressure’ from the east way before the Roman occupation. When the Romans arrived, some two thousand years later, the language border had moved considerately westwards, especially in the northwest of England.
At the end of the Roman Empire, wealthy Welsh families were proportionally over-represented among the rich and powerful in Britain and some of them must have become very rich. 
Typical for instance was that tax collecting was outsourced by the Romans. The collection of taxes was auctioned. The richer southwest Welsh lords and merchants had more possibilities there. Ports could easily be controlled, income taxes had not been invented yet, so the main tax sources were import and export taxes, harbour and transport taxes and excises. The British southwest ports were the main gateway to the rest of the Empire. These taxes were used to pay the administration and army.
Late Roman Britain was characterized by a small but very powerful upper class, organized in families. All power in Britain was concentrated in that tiny class of landlords. This was very much the situation all over the Empire. It was probably caused by Roman inheritance laws.
The oldest, classic heritage laws in western Europe dictated that when the father died, all
his possessions had to be split among his sons. There was a limit to that rule : when a parcel of land became too small to
sustain a family of farmers, the land was not split further. In those cases the eldest son inherited the lot. In the German
speaking part of Europe this person was named ‘king’ or ‘koning’. The oldest meaning of
‘king’ is heir. A word derived from ‘cun+ing’ meaning : ‘cun’, ‘cunne’,
‘conne’ (‘Conan’?) = son , word related to ‘kin’ (next of kin), kind (child in German or
Dutch) and ‘-ing’ means family, related to, parentage. Powerful and rich families sometimes misused this
exception to maintain power.
We should not imagine that each British family of landowners ruled one enormous domain, say, the size of half a shire. More likely was that a single family owned large possessions spread over all of Britain. Each separate domain would have been ruled by the local branch of the same family. Nevertheless, they formed a single family with a powerful pater familias on the top. This man acted like a modern CEO, coordinating the management of the local heads of family. In many cases, he would have been elected by the family members. Power derived from land ownership and this was protected by Roman law. In fact, without the reinforcement of the law by the Roman administration and army, those families had a great problem as they were too few to defend effectively their possessions. All over Britain, there must have been no more than a few hundred ‘names’. The gap between the rich and the poor must have been great. This social organization managed to produce a massive amount of wealth for those with property. Britain was known to be a great exporter of all sorts of merchandise in the third century.
The social situation in Britain during the 4th century and early 5th century is important to understand later events.