Archaic bullshit, based on a lack of real history and contradicted by all the archaeology. About ten thousand German mercenaries in a disarmed population, took over in the eastern provinces, whereas (probably) British ones from north of the Wall took over in Britannia Prima.
Which, apart from the fact that we got shot of the Roman bureaucracy in 410, in why the Germans took so much longer to take over in Britain than anywhere else in Western Europe.
We know it's true if you can find it on the 'net.