Well Brytenwalda is based in Britain before king Arthur united whole Britain(at least I think)There were 100 kingdoms in Britain then,also Vikings would mostly sail to Britain to raid and pillage but I never heard that they established a Kingdom there...Maybe they did if they did most likely they would have been located somewhere near the the York(Scandinavian York) I know that few times Vikings invaded England mostly in late 9th century...Sorry for quick history lesson
so to answer your question It is possible that there is Viking Kingdom in Brytenwalda but I Dont know their Kingdoms name so the closest thing to Vikings would be Saxons! They were same type as Vikings just from Northern parts of the Germany not Scandinavia.So if you are looking for something close to Vikings join Saxons!