Has anyone worked on using KAP for later periods? The Boy King had some nice correspondence charts in the back, such as 531 AD = 1240 AD. That part is easy.
I'm more curious about character creation. So, just for example, say I wanted to set up a campaign in Essex c AD 1240. What about cultures? If using the BoKL, would you use the French for Norman culture? If we really scrolled the clock back to the Pendragon era, then those who would later be known as Normans would be the Danes. So should you use the Danes? Perhaps Danes for Attributes & Statistics? Of course, between the Pendragon era and the 13th century, there was plenty of intermarrying between the Danes & Franks in Normandy and Normans & Saxons in England after 1066.
Society is certainly feudal (early middle ages, I believe). Religion should be Roman Christian, though there might still be some holdout British Christians around.
Cymric always had the best chance of being chivalrous. So does the Cymric better reflect a noble culture? I would greatly appreciate your feedback on this. Thank you in advance.

In my humble opinion, you'd be better off writing your own. The Cultures and culture modifiers in Pendragon are steeped in Pendragon Lore. Sure, you can probably rip off Occitaneans for the people from Southern France, French for Northern France, and then you get stuck with Anglo-Normans. What should they be in your campaign? Especially when you get to the character traits, it is clear that Arthur's rulership of Logres in general boosts the chivalric tendencies of the Cymri under his rule, as shown by the 531 AD starting point (of previous editions at least, I don't know if that has been re-published for 5th Edition). Hence, the Cymri have the best chance for Chivalry not because they are Cymri, but because they have the Legendary King of Chivalry that they try to emulate.

The Medieval sources also had a habit of slandering other peoples in order to elevate their own (patron). So that is another thing. Me? I'd say that the knights of England probably had a heck more in common with knights in France still in 1240AD than with their own serfs. At that point they still shared a language, too, although it was a different dialect. I'd say that the English knights of Edward III at the start of the Hundred Years War still thought themselves more or less the same kind of guys as the French knights on the opposite side. I'd be hard pressed to come up any game mechanic difference statwise or even in traits, based on something as nebulous as national cultures at that stage. Of course, you can come up with stuff on your own, if you wish to introduce such, but being a lazy GM, I'd rather just have 'Cultures' as a background flavor, rather than anything granting you bonuses and penalties.

Has anyone worked on using KAP for later periods? The Boy King had some nice correspondence charts in the back, such as 531 AD = 1240 AD. That part is easy.

The GPC is set to imitate the whole Medieval era
The Twilight Period is the Hundred Years War, for instance


I'm more curious about character creation. So, just for example, say I wanted to set up a campaign in Essex c AD 1240. What about cultures? If using the BoKL, would you use the French for Norman culture?

I'm not exactly sure of your question here
The Cymric culture is the analogue of the Normans

Thank you, Greg! That Cymric is the analogue to Norman was precisely the answer to my question.

Morien, you had some great suggestions, too. Thank you.