The evidence he cites for his thesis largely comes from the new science of 'genetic archaeology' which has discovered through paternal DNA that a significant majority of those who live in the UK today are descended directly from the Ice Age hunters who first came here it seems from the Iberian peninsula. The figures are 88% of the Irish, 81% of the Welsh, 79% of the Cornish, 70% of the Scots and 68% of the remainder of England ... So despite invasions of Romans, Angles, Saxons, Jutes, Norse and whoever the ethnic makeup of the UK has hardly been altered.
One of the side effects of this research is to massively undermine ideas of Celtic difference. It suggests that there is precious little such thing, if anything any difference predates (the) Celtic retreat from Anglo Saxons by 5,000 years.
I would guess that the numbers would drop even further in the Danelaw, perhaps down to 60%. Still, that is much more than folks there would estimate, I'm sure. They see themselves as less Celtic.
It's not much of an issue in America, where we are so clearly ethnically mixed that it's hard to see what the fuss would be about. We have some curiosity about British development because our institutions have British origin. The history of the United
States is usually told from the perspective of British-style government landing on the eastern coast of America and moving west with settlement, being influenced by many other strains of people - indigenous, African, and other European, mostly - as it developed. Yet American ethnicity is only minority British.
How those British institutions developed before they got here is viewed quite differently by English traditionalist/conservatives and American Anglophiles. The blood-character is of greater importance in the Old World, where the English impression is that they are much more Norman, Danish, and Saxon, than all those wild Celts to the west and north.
To Americans it would seem less of an innate ethnic issue and more of an institutions issue. That there remained a considerable, even dominant Briton-Celt bloodline would matter less than the expansion of Norman-Saxon custom and language.