Was there any actual need for this?
The TV series worked as a (often slow paced / sluggish) series which focused on the life of Uhtred (son of Uhtred) and his journey during the 9th and 10th centuries against a backdrop of the wars between the Vikings and Anglo-Saxons and the (eventual) birth of England.
The film throws this all away, using Uhtred as an anchor to focus on the birth of England side of the story. It doesn't work. The film takes on way too much for its 2hr runtime, is rushed and the passage of time (which was crucial to the series) is cast aside as it tries to sprint to its conclusion.
Most of the known cast from the series feature - but their point in being there is almost pointless as they don't offer anything other than familiarity to those who watched the series.
The story was repetitive and the 'epic' battles were embarrassing given the 'armies' consisted of about 30 people.
Other than a vehicle to make money, there really doesn't seem any point or need for this; it feels like an add-on and ruins the well structured conclusion to the series.