We could go along with that narrative if you want to go along with the one that involves the creation of the English, Welsh, and Scottish national identities and modern Britishness through mass genocide during the so-called Dark Ages, since that one's about as accurate. If you're going to be wrong, you might as well be wrong about everything.By inviting a foreign nation to kill them through war and disease, drive the survivors into the wilderness, and repopulate the country with poor migrant workers?