DNA research has overthrown the paradigm of British prehistory as a series of population replacement events, which informed fiction and popular books on the subject. This false model is of 'races' such as the Beaker People, the Celts and the Saxons coming into the country and replacing/displacing previous populations. The British identified as an Anglo-Saxon people, and identified the inhabitants of (say) Wales as displaced pre-Saxon populations.
DNA research proves this model to be very inaccurate. Instead Britain has a population which originated during the Mesolithic (from the Ice Age population centre in Northern Spain) and then assimilated repeated minor waves of immigration, such as the Saxons, which each contributed about 5% to the population (a similar proportion to 20th century immigration). You may see this as politically positive - we are not survivors of a history of genocide and racial supremacism, we are survivors of constant assimilation and inter-relationship. Also modern immigration patterns are consistent with thousands of years of population flow, rather than being an unprecedented threat.
A second issue which I'd like to post about another time is the pseudo-Darwinian 'genocide' model which was accepted for so long: I think it is no coincidence that it is a warrior model, a conquest model, a masculine imperial model. I also think it is no coincidence that it arose just as people from Britain (and other European countries) were actually displacing and destroying indigenous populations in other parts of the world. But we now see that it is not the model of how Britain itself became populated.
I should also say that if the DNA had proved we were the victors of waves of genocide, the value system which says genocide is bad would still be just as strong. However, it is nice to see that as usual, reality has a liberal bias.