Just a little overview and myth-busting. Most tribes have a continuity belief, that their people have always been there, or were there first. When DNA was extracted from Cheddar Man, he was found to come from the rare mtDNA Haplogroup U5, and as there were people nearby with U5, including the local schoolmaster, the news sources put it about that this was a remarkable example of 9,000 years of continuity in that spot. Brian Sykes was looking for drama, perhaps, but it ends up being misleading in that regard.
U5, along with U4, and U2, were common in the first group to resettle northern Europe after the glaciers retreated about 10,000 BC. These were the Western Hunter-Gatherers (WHG), and the strongest remains of their genetic signature is now among Finns, Estonians, and Sami people, though not common even there. Doggerland was still above ground for the next 3-4000 years, so Britain was not isolated from the continent in those days. There is now some thought that Doggerland may have persisted as a land bridge, though a less hospitable place to live, for even longer, with a remarkable date of 3800 BC suggested. Still, it was sparsely settled. These were not easy places to live.
Farming started spreading out of the Levant about 7000BC but did not reach Britain until around 4000BC. These were the Early European Farmers (EEF), and they almost completely replaced WHG peoples in a few centuries - more than 90% turnover. When one population "outcompetes" another that thoroughly, some violence is assumed. However, both populations were usually violent with each other before contact, and we needn't assume it was uniform invasion and genocide. They would have had different tools and ways of surviving on the land and may have lived near each other for long periods, not needing the same space. But when famine or natural disaster hits, one tribe is going to be better placed than the others to survive. If climate is changing or a region has been overfished, overhunted, or overfarmed then such advantages are intensified. Even in good times, if the women of one tribe average four children surviving to adulthood while the next tribe over averages two, that will make a large difference in a hundred years, and an overwhelming one in three hundred.
At the time we are living we measure success by what is happening to ourselves personally, or perhaps our clan. In most times and places we would call what the men's lives are like as more successful, even with the dangers. Yet if we measure success by genetic continuity, of sending something of yourself down through history, women's success is enormous. Men's genetic lines die out all the time, through warfare, the dangers of hunting and fishing, or being left out in polygamous societies. Women's lines persist, even after conquest. Not all of these matings would be violent or unwelcome, just survival choices. They might be unattractive lives of slavery, but humans have generally accepted that over the death alternative. So mtDNA lines can hang around a long time, well after any semblance of a tribe has disappeared. Yet even those can die out.
If your tribe is on the receiving end of the violence, it doesn't much matter whether it is by a tribe that separated from yours a hundred years ago that speaks your language or new arrivals that split off from your ancestors 40,000 years whose language is unrecognisable. If modern experience, such as the Taino vs the Caribs, or Wampanoags vs the Narragansetts, or many resentful tribes subdued by the Aztecs is a natural trend, the invaders may have been welcomed for their trade and help against long-held enemies, but regretting the short-sighted welcome sooner or later.
So EEF shows up and WHG - which is Cheddar Man's group - almost disappears in the genetic signature. EEF prospers and builds Stonehenge, first as a circular ditch with a circle of small stones or perhaps timber henges in it, but eventually the dramatic structure with bluestones that we see today. Just as they were finishing up, they get outcompeted as well by another group of hunter-gatherers, the Beaker culture, and are invisible by 2000BC. It has been controversial how much of Beaker is Corded Ware, and how much both owe genetically to the Yamnaya*, which we would call the Eurasian Steppe people, a branch of Indo-Europeans. Migration vs cultural diffusion remains an issue on the continent, but genetic research reveals that there is no question that large numbers of people moved, not just some technologies adopted, nor an elite governing a large population while remaining separate. They replaced EEF within a few centuries, upward of 90%. Interestingly, they continued to use Stonehenge. My thought is that if your gods are sky and earth, impressive cursus and megalithic monuments are going to look just fine for your purposes, especially if you have seen your neighbors putting them to use for a few generations.
Replacement of males is usually more thorough, and the quicker it happens the more likely it is to be a result of war. It also means they likely killed the male children as well.
You will note that this is the second 90+% population turnover since Cheddar's day. The odds are growing slimmer that the current schoolmaster in Cheddar Gorge is any kind of close relative of the fossil find. While EEF had replaced WHG on the continent as well, it was less thorough, and the Beaker people subsequently replacing EEF was less entire as well. Women passing on U5 mtDNA were not common, but there were many more outside Britain than in. The schoolmaster's female uniparental line was more likely to have come in after, even as late as the Viking invasions. It was not mentioned at the time, but he wasn't even born in the town to begin with. It was all a nice piece of theater based on some real data.
*The pendulum has swung back and forth as to how much of Corded Ware was Eurasian Steppe invaders, with some recent research suggesting that they were themselves invaded, not the aggressive ones - as if any peoples of that time can be counted on as examples of non-aggression. However, Nick Patterson has hinted that research just about to come out is going to push the pendulum back, that some Corded Ware DNA is related to steppe pastoralists far to the East. They are thus more likely to be Yamnaya themselves.