Parts of England came to be settled and governed under Danish custom. What was that area called?

Explication

The name marks a region of England where Danish incomers were numerous enough that local law and custom followed Danish practice, and it covered much of the east and north rather than the whole country. Wessex is the near miss here, because it is the other kingdom name pupils meet in the same chapter, yet it was the English kingdom that resisted the Danes rather than the region living under their law.