"Wales was another area spared from invaders. So why measure the genetic makeup of Britain which includes those areas invaders avoided? "Stupid? I think not. The DNA results support or disprove accepted history.
2-3 different DNA studies by different researchers have failed to find any clue of Picts in Scotland as an example. They are all the same people, just different culture.
What do you think Offa's Dyke was about?
The generally accepted theory about much of the earthwork attributes its construction to Offa, King of Mercia from 757 to 796. The structure did not represent a mutually-agreed boundary between the Mercians and the Kingdom of Powys. It had a ditch on the Welsh (western) side, with the displaced soil piled into a bank on the Mercian (eastern) side. This suggests that Mercians constructed it as a defensive earthwork, or to demonstrate the power and intent of their kingdom.
I recommend you read this book, Origins Of The British
2-3 different DNA studies by different researchers have failed to find any clue of Picts in Scotland as an example. They are all the same people, just different culture. I would expect genetic studies of Scotland would show up as primarily as Scots followed by Irish, Norse, Normans, and lastly by Germans. This can be derived from genealogical means. I would not expect Romans in the bloodline. They built Hadrian's wall to shut them out.
My complaint is that a genetic study of Britain as a whole is defective. The area now called Britain had varying histories of conquest in England, Scotland, Wales, and Northern Island. Would one expect more German genes in the English than in the other areas? I would. Why tell us only about the 30 percent. What is the 70 percent?