What is the setting of The Wicked Day by Mary Stewart?

The story is set mostly in the British Isles, and some in Brittany, which is the home of Bedwyr and located on the coast of what is now France. The time is the medieval period of England, when transportation was by horse and wagon, and communications were through couriers carrying letters sealed with wax. England was divided and ruled by many kings, and King Arthur united them all for a short period of time, under his rule as High King, and enforced by his hand-picked knights, or Companions, as this novel calls them.