If I understand this correctly …
The original inhabitants of Britain who arrived after the last Ice Age around 8,000 BC and who existed through the Stone Age down until around 2,500 BC were an olive skinned, blue eyed Mediterranean race of people. They were basically like modern day Sardinians.
Around 2,500 BC, the Bell Beaker people arrived in Britain from the lower Rhine. These people had copper tools and weapons and were horse mounted archers. They spoke an Indo-European language and dominated Britain through the Bronze Age until around 800 BC. These people conquered and wiped out 90% of the Stone Age Neolithic farmers, cut down the forests and created a world of farms. The vast bulk of our ancestry is derived from this group.
Somewhere after 800 BC, the climate of Britain began to improve and there was an influx of Celts into Britain who expanded out of what is now southern Germany, Austria and the Czech Republic. These people brought the Celtic language and gods and iron tools and weapons. The Iron Age may have begun in Britain before their arrival through trade with the continent, but these people had about a 10% genetic impact on Britain. They did not replace the preexisting Bronze Age population. Instead, they intermarried with them like the Anglo-Saxons and the Britons adopted Celtic languages.
When the Romans arrived under Julius Caesar in 55 and 54 BC and later in 43 AD under the emperor Claudius, the Britons were speaking Celtic languages and worshiping Celtic gods, but this Celtic culture may have only existed in Britain for a few hundred years.
The Anglo-Saxons arrived in post-Roman Britain after the Romans pulled out in 410 AD and specifically in England may have had about a 10% to 40% genetic impact on the British population. As with the Celts before them, the conquered Romano-British population adopted Old English and the Germanic gods. The culture and language changed and there seems to have been some kind of apartheid-style system at first in which Britons and Anglo-Saxons lived in their own communities. Eventually, this broke down and the two groups merged.
The Anglo-Saxon kingdoms converted to Christianity around 600 AD. Irish missionaries operating out of Iona converted the Picts and the Northumbrians while the Roman mission sent by Gregory the Great took root in Kent and spread to the other Anglo-Saxon kingdoms.
The Vikings and Normans both conquered much of England and Scotland, but like the Romans didn’t have much of a genetic impact although it is hard to tell because the English and Danes were so closely related anyway.
The upshot of this account of British history is extreme continuity. Nothing much has changed since the arrival of the Bell Beaker people in Britain in 2,500 BC. There have been cultures, religions and languages which have come and gone. There has been conquests from adjacent areas of the continent by various peoples descended from the same founding stock.
At least that was the way it was in Britain all the way down until the disaster that has been fostered by liberals since World War II who have let radically different peoples from all over the world settle in Britain and encouraged them to demographically take over the country.
Note: As for the American South, it was an offshoot of the above that began four centuries ago and was relatively unchanged until recent decades.