When the Romans departed in AD 410 they took with them most of the British warriors as conscripts leaving Britain vulnerable to the attacks of the Irish and Picts. Gwrtheyrn (Vortigern) a British chieftain paid mercenaries to help his people defend themselves. These paid soldiers were the Angles, Jutes and Saxons from North of the Rhine. They took their opportunity to exploit the power vacuum in Britain and killed the British Chieftains during a meeting of the respective leaders. It was called "The night of the Long Knives". The Anglo-Saxons eventually mixed in with the weakened British and settled in most of today's England. The British held on to their hinterlands of Cornwall, Devon, Wales, Cumbria and Strathclyde. The Picts settled in Scotland while the Scots went to Ireland...confused?!
In AD 731 the English churchman and historian, Bede, wrote "There are in Britain, in harmony with the five books of the divine law, five languages and four nations - English, British (today's Welsh), Scots (today's Irish), and Picts (today's Scots). Each of these has its own language, but all are united in the study of God's truth by the fifth, Latin."
To add to this melting pot of cultures and languages came the Vikings. They came from similar roots to the Anglo-Saxons, their languages would have been closely related. Saxons, Welsh, Irish and Scots all felt the wrath of the pagan Norsmen who raided Britain for 0ver a century. They occasionally settled in places on Anglesey, the Irish coast and York was a large Viking settlement.
Then came William the Conquerer and the Norman conquest of most of Britain in 1066. The Normans were Vikings who had settled in Normandy. They governed the Saxons and French became the official language of the court in England. The Normans didn't conquer Wales or Scotland to the same extent, but a century or so later, and once they had assimilated with the Saxons to become the English, they eventually achieved dominance over the Britons and the Picts. The English eventually came to be known as British as well.