There has been many foreign invasions onto the British isles throughout history (Roman, Anglo-Saxon, Normand). Did this in practice mean that the populations that lived there before the Romans were largely replaced by the different waves of invasions and migrations? Did the groups mix to such a degree that most modern Englishmen has an equal amount of Brytons, Angles, Saxons and Normans in their ancestry? Or has the bulk of the population largely remained the same with a minority of ethnic groups mostly changing the culture of original inhabitants?

Additionally, how does ancestry differ across class? I suppose most nobility/upper class people have mostly Norman ancestry, but does the middle class have mostly Anglo-Saxon ancestry with rural or lower class people having mostly Brytonic ancestry? Is the majority of non-upper class population mostly mixed?

Thanks in advance.