To say that it was merely a cultural takeover would be a sevre understatment.
The entire Jutish population had to flee due to an increasingly inhosbittable homeland. David Starkey writes that in recent DNA surveys in the midlands and the east over 90% of the male population was displaced. These people were not merely warbands but settlers looking for land, they were obliged to force the natives out. It is estimated that about 200,000 Saxons Angles and Jutes flooded into Britain within the first fiftey of so years. Also, the cultural assimilation of which so many speak deos not make sense, for on the mainland Germanics took to native Roman ways by learning Latin and keeping local offices and cities in tact, in "England" no such occurance took place, the Germanic settlers replaced the official language with Old English, on the mainland Germano-Romans appeared, a society which championed the idea of a Christian Rome. In England the Germanics did away with the religion and laws.
I have read peices about cultural assimilation, yet thay are backed up with weak evidence such as Celtic curves appearing on manuscripts, compared to the veidence against this, its a weak link.
Then what about Offas Dyke? What is that construction of wim? Its more a symbol telling the Britons of Wales that they no longer hold Britain and they are now the Barbarians. Bede makes no mention of large British populations intermingling with the Germanics, yet he deos make refreance to the peoples of the English Kingdoms and makes no mention of any Celtics.
Then there is Gildas, even he deos not show any sign that the British held onto their land, he lements the loss of his ancestral homelands.
I see no real argument either from modern day findings or Historical records.
Why these Germanic peoples acted so differently to their mainland bretheren may be due to the fact that the Britains were no push over like you said the last British Kingdom (Elmet) in what was to become England did not fall untile the end of the 570's.
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