I absolutely adore this! I have been toying with something similar in my head, and it is fun to see your conclusions. I am not British myself, but I did put this idea (https://imgur.com/a/lgmcOWv) together. I think the use of Rheged and Elmet are great, and I defer to your understanding of demography and configurations. I worked on a bunch of the flags (updating them on Illustrator, &c.) and so maybe it can be of use to you! Let me know if you want .svg files