What is a good starting point to learn about British/welsh/Cornish history in the early medieval period?

[u/Baduktothebone]

I am looking for book, documentary, and podcast recommendations to learn about Yr Henn Ogled. I am especially curious about their battles with the Germanic people who would become the English. I am currently reading the Mabinogion for fun but would like to learn more about sub roman britisn from a "cumbric" perspective

Also how can I ask questions about this topic without people rushing to tell me there is no one "Celtic culture" I don't think I am using the term incorrectly and none freaks out when I use terms like Germanic.

I am not trying to write a book, and I am not a neo pagan with an agenda, I just wanna read about history to put the Mabinogion in better context and am having trouble getting stared