At the time you are talking about the various tribes, from wherever they came from called themselves those names. I didn't even bother to mention the Iceni, who were also of Germanic origin, pretty much ALL of England was held by invaders. There were no "Picts" That name is a Roman invention from the 3rd Century. The tribes that inhabited that area were the Damnonii and Cornavii. Both Celtic. The Welsh are also Celtic. Like I said, there never was a native Briton, the island is inhabited by invaders from day one.
As far as the volcanic evidence, it is pretty well established that the Sun was in a cool phase through the late 5th, through the 6th century. That is going to lead to lower crop yields, toss in a volcanic eruption or two, and the corresponding crop failures leads to widespread starvation.
These are all documented by the catholic priests of the time, as well as the Chinese scholars and bureaucrats of the east.
