The Romans had left the Isles. The isles knew chaos. They wanted a leader - there was none to be had.The Plague of Justinian was driving the Angles, Danes, and Saxons to flee into the British Isles. A little known captain rallies the remaining forces, defeats the enemy at Badon, and becomes "King Arthur".
The plague remained and Arthur knew that it would cross the Chanel. He and his knights set out the quest of the Holy Grail - a holy relic to save the Celtic-Britannia from the plague and ensure that the Angles, Danes, and Saxons would not return for another strike. The "grail" was found, but there was a cost for the use for such a magical item - it was a one-use item that cost the life of the user. Arthur consulted his knights and Merlin. As they argued, Arthur used it.
Celtic-Britannia was saved, but at a cost.
Arthur is succeeded by his nephew Mordred as "High King". Mordred marries two princesses - Aeiffe of Ireland and Cywyllog of Scotland - creating the union of the isles. High King Mordred is succeeded by his son King Melehan. Melehan marries a Saxon Princess - best way to keep them from invading again continuing the dynasty to the Viking Period.
In 865, overcome by cowardice, Morcar II flees from the battlefield leaving his crown behind. Civil war breaks out, Danelaw takes a good part of the isles.
In 1066, William of Normandy takes the throne of England.