In Assassin's Creed Valhalla, Ravensthorpe is located in the historical kingdom of Mercia.
Ravensthorpe is Eivor's settlement in England and becomes the organizational center of the Viking clan after the story moves from Norway. The settlement is built in a region associated with Mercia, one of the major Anglo-Saxon kingdoms of the period.
Valhalla presents England as a collection of competing territories, alliances, and rulers. Eivor travels through areas connected with Mercia, Wessex, Northumbria, and East Anglia while helping form political relationships and confronting enemies linked to the Order of the Ancients.
The exact historical geography is simplified for game design, and Ravensthorpe itself is fictional rather than a documented medieval town. That makes the kingdom name the most specific answer: it identifies the regional setting without confusing the settlement with a real historical capital. Wessex is especially easy to choose because Alfred the Great is central to the English storyline.