In the last map, I noticed that Bernicia had a capital in Durham, but I'm not sure if the Anglo-Kentish have one, Lunden maybe? Also, is either one stronger than their counterpart or are they more or less equally subordinate to Brythain?
The three english kingdoms have their own separate kings & courts, and ergo separate capitals. Northwic for the East Angels, Coluncaester for the East Saxons, Kantaburh for the Jutes, and the creatively named Wihtwaraburg for Wihtwarans.
Lunden is under the direct authority King of Brythain, and is his court in his capacity as Bretwalda, although he lets the ældarmenn of the city handle most of the day to day governance.
In terms of dominance, the East Englia and the Kant are the big players, and they vie for dominance over the East Saxons. Wihtwara is mostly ignored by their mainland cousins