Largely because of the Anglo-Saxons which migrated there after the Roman period from Denmark and Northern Germany that first aided the Britons in conquering the Picts, then turned on them and conquered the Britons. They ruled most of the area that is now England for about 500 years, introducing Germanic language and culture there. They formed several kingdoms including Northumbria, Mercia, East Anglia, Wessex, Essex, Sussex, and Kent. These kingdoms united in the 10th century under the king of Wessex, Aethelstan, and the land was named Angland, which is to say, England. The Anglo-Saxons dominated English culture, though they did mix somewhat with existing Roman-Celtic culture. England was conquered by the Normans in 1066, after being significantly weakened by aggression from Viking invaders, ending the Anglo-Saxon period. The Normans were originally Viking raiders as well, but had settled in the region that is now France and intermixed with Frankish culture.

During all of this, Celtic cultures kept getting pushed back, and many Celts were the subjects of oppression. This is why there seems to be very little Celtic culture in most of Modern England, while places like Wales and the Isle of Man, as well as Ireland and Scotland, are much more Celtic in nature.

Largely because the depopulation of Eastern England caused by plague, and the de-urbanisation and rapid collapse of Roman culture led to Anglo-Saxon culture very quickly becoming dominant in England and southern Scotland. Peter Salway in "A History of Roman Britain" suggests that the fertile parts of Britain had become completely romanised prior to the arrival of the Anglo-Saxons, Hence celtic culture had declined significantly if not terminally in these regions long before the arrival of the Anglo-Saxons. Hence, although the population of Britain genetically was pretty much still celtic, the culture was entirely Roman. The collapse of Roman culture led to it being replaced by the Germanic culture of the immigrants, so society and language became English. The marginal lands and the fringes of England, Northern Scotland, Cornwall, Cumbria and Wales, remained largely celtic as Romanisation was far less established.