Almost all of the British Isles is inhabited by people with a mixture of origins. You could certainly say that there is a large influence of Celtic origins in Cornwall, less so in Devon. And have you considered the proportion of Viking origins in Devon? Most coastal areas of the British Isles have some Norse (Viking) influence.
"The English Riviera" (note spelling) is a very recent invention, less than 170 years old, and the name stems from "The French Riviera" beloved by very rich British people who could afford to visit the South of France. You are asking about origins at least 1000 years old, and mostly much older.
You also need to think about the Normans - it was they who ruled the land (England in total) for a few hundred years, built castles, reinforced European religious practices, built cathedrals etc.
Oh, yes - there is much more to think about apart from Celts and Anglo-Saxons. Oh, gosh, I forgot the Romans!