Cornwall (and Brittany) are "the west". Cornwall includes the entire southwestern peninsula of Britain. It is famous for its rich tin mines and its close political connections with Brittany, whose settlers have been populating and dominating the northern half of the peninsula for a generation.
The northeastern half of it was the Duchy of Cornwall ruled over by Duke Gorlois. After his death, King Uther never appointed a new duke. The duchy consisted of the counties of Tintagel, Devon, and Jagent.
After the death of King Uther, King Idres conquered all three counties, incorporating them in the Cornwall Kingdom.