In Saxons, Vikings, and Celts (aka Blood of the Isles) by Bryan Sykes, he argues that the the British Isles do not have a significant genetic contribution from any invasions, and that the inhabitants are genetically similar to the prehistoric island inhabitants.

I'm curious about this book, because it's an interesting idea. But as far as I know, he didn't submit it to a journal, and there has been no peer review of his findings. He just wrote a popular science book about his own original research challenging the conventional scientific view that the British Isles have had a series of invasions. I'm also worried because it turns out that this guy has a book about bigfoot dna, which is ridiculous and makes it look even more like he's a crank.

Here's my specific questions. Is the history in the book good? Is the genetic science in the book good? Was his research sound, and does it support his conclusions? If his research is bad, is the hypothesis that the Isles have had little genetic contribution from later invasions a good one? Or is it a fringe idea? Are there legitimate historians that argue this?