Stupid. Stupid. Stupid. These invasions took place largely in what we call England. Rome did not go into what we call Scotland, Northern Ireland. 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