How old is English?
 Pierre Flobert. "Latin/Frankish Bilingualism in SixthCentury Gaul: The Latin of Clovis." Bilingualism in Ancient Society.Ed. J.N. Adams, Mark Janse, Simon Swain. Oxford: Oxford University Press, 2002. 419430
Social pressure and promotion
Normally, you would adopt a foreign language as your own if it provides a lot of added value. Suppose that all Britons spoke Brythonic before the coming of the Anglo-Saxons. What added value could the English language provide to them?
Roman society had been reasonably open. Roman law was applicable to every citizen. The nobility of one's family was not mandatory to access the highest ranks. Careers were very much possible.
Many Roman emperors came out of 'nowhere'. The British co-emperor Constantine III is such an example. He was qualified by his contemporaries as 'a man without nobility' ('sin nobilitas', or s.nob.). We have no idea from where he suddenly 'popped up'.
The knowledge of Latin was necessary if one wanted to make a career in the Roman administration, or in the Roman army or, later, in the Roman Church. Latin opened the doors to promotions. Speaking Latin in daily life meant that you had ambitions or that you had become a part of the upper-class.
In contrast, Anglo-Saxon society was completely closed. Power and rank was and remained a family matter. Climbing the
social ladder, the ascent to power, was very limited. Birth determined what rank you had, not the knowledge of a
language, not even knowledge in general. Such a social system was typical for the aristocratic age.
After Clovis had achieved his conquest of Gaul in 507 AD, Low Franconian, the ancestor of Flemish and Dutch, remained
the language of the newly established Frankish aristocracy. The Gauls themselves continued their language transition to
French. Texts, contemporary of Charlemagne, reveal that the Frankish aristocracy had the habit of sending their sons to
Brabant (Belgium) "to learn proper Frankish". Low Franconian was the language of Charlemagne's court. One spoke in
Gaul, at the time, many languages. Low-Franconian, however, became a language that could be understood by the whole
aristocracy. This Low Franconian, which can also be called Old Flemish, became the "lingua Franca", the language of the
Francs. So, lingua Franca is not French. After the death of Charlemagne, the French aristocracy gradually adopted
French as their native language. Low Franconian remained the aristocratic language in France in all for some 300 years
(507 - ±814 AD).
Speaking English as a daily language provided therefore little advantage. Why would the British change their language? This would improve by no means their daily situation. On the contrary, speaking English as if it was your own language, could make other people suspicious that you were misrepresenting your actual rank in society.
After the fall of the Roman Empire, (private) schooling was no longer fashionable. Only the Catholic Church would continue to teach Latin. In addition, there are no indications that the Anglo-Saxons organized some sort of English language courses.
After the Arabs had conquered Egypt, they organized courses in Arab and Islam. The Muslims considered Arab to be the
language of God. Attending to those schools was compulsory for the Egyptians. If you did not, you had to pay special
taxes. This resulted in a language transition from Koptic to Arab, but transition time took more than a thousand
The statement that the Anglo-Saxons spread English because they were in power can therefore be dismissed.