I've added a section that makes the Struggle for Britannia mod inherently compatible, though the High King mod should be loaded AFTER the Struggle for Britannia mod.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2850397747

Load Order
X1 Mod
X2 Mod
Struggle For Britannia
X3 Mod
High King Elective of the North Sea
X4 Mod
X5 Mod


I've basically made it so the Kingdoms of Mercia and Northumbria merge into England as they should with the Struggle For Britannia mod.
If you don't have the mod, nothing happens because the Kingdoms don't exist.

It also appears that the original mod developer as stopped working on the mod.
So I'll leave in the code for now, and then hope to make an additional mod in the future for a Struggle for the North Sea mod as it were, incorporating all the cool things I've added here.