The idea that the Anglo-Saxons "drove out" the Celts is an oversimplification. We now believe it was more of a cultural conquest than a population-based one...Celtic culture survived in the west and north but England was Anglicized. However, it's still the popular view and is hard to shift.

Also, both Northmen raids and Anglo-Saxon migration may be explicable by...climate change.

We have records of the Romans growing warm weather grape vines in Britain and other records that indicate that the climate cooled significantly at the end of the Roman period. This would have created challenges for farmers in the colder parts of Europe...which include Germany.