As you can probably guess from the title, the POD of this timeline is in 793. In OTL, there was a Viking raid on the island of Lindisfarne, also known as the Holy Island, in 793. While the British people were scared, as proved by Alcuin of York's quote "Never before in Britain has such a terror appeared," they never retaliated by declaring war against the Vikings. In this timeline, however, the people of the British Isles decide to fight back after the raid on Lindisfarne. A week after the raids, Northumbria declares war against the Vikings. The Vikings easily invaded Northumbria, and they surrender less than a year later on May 23, 794. From there, everything went downhill for the remaining British kingdoms. On November 20, 798, the British Isles became fully united under the Vikings after Connacht surrendered. On the same day, the North Sea Empire was established with its capital in Trondheim and Markus Viktor as its King.
That is when things start to change - and drastically.