That being said, while it's much more unlikely than probable, it is your fantasy and you can do whatever you like as long as you are describing your myth. But if you are trying to describe the arthurian myth as envisioned by the populace, they will be (apart from specific characters like Palamedes who are not even that well describe and vary according to the source) welsh britons.
And again, there is a world of difference between "plausible" and "probable".
You are doing a fantastic job about these characters, and the illustrations are really nice, nothing shocked me there, this if YOUR fantasy and it's incredibly cool (and thank you for this), but this answer trying too hard to justify your choices and using articles about probabilities in what I believe is really a wrong way slightly bugged me to be honest.