This page of the Assassin's Creed Valhalla guide provides an explanation of the England storyline ending.
Eivor travels to Hamtunscire to help his friend take over power. Guthrum demands the crown of Wessex, which King Aelfred refuses to accept, and gives the Normans an ultimatum: they have one day to leave Werham or he will take the city by force.
Eivor and Guthrum are not intimidated and decide to burn down Werham and then deal with King Aelfred. They attack Cippenhamm under cover of darkness, in the midst of a feast that Aelfred wants to hold to celebrate the birthday of his saxon God. You will be assisted by all the allies Eivor has managed to gain across England: Ljufvina and Hjorr, Tewdwr, Erke and Stowe, Broder, Finnr, Vili, Hunwald, Geadric, Bishop Deorlaf and even young Eluric (son of Birstan and Alfida).
When Eivor wants to find Aelfred and confront him, Goodwin, Aelfred's trusted man, stands in his way. The man reveals that his king is long gone from this place, and that the Christian feast was just an excuse to lure them to this place and give Aelfred time to gather his allies. A fight ensues between Eivor and Goodwin, which Aelfred's faithful servant could not win.
The city of Cippenhamm is practically flooded with King Aelfred's men, but Eivor manages to capture it with the help of his allies. Aelfred's forces are shattered. Guthrum has finally got the crown and is now celebrating his triumph at Hamtunscire.
King Aelfred has been dethroned, but the price of this victory was enormous. During the final battle, three allies of the Raven Clan and friends of Eivor and Guthrum fell: Soma, Hjorr and young Hunwald.