Again, this is Alt Leaders. Grouping several civs together because of geographic location does not fit the bill sadly. Byzantine Leaders are not alt leaders of Rome. They are not alt leaders for the Ottomans. But then we enter the tricky area of "Are the Turks an alt leader for the Ottomans?" I think it's up to the mod creators to decided that. Personally, if it's a situation of Denmark-Norway, that should be counted separately. The UK however...
Alternatively, what could be done is that Civs that evolve into a greater collective could all share that same option in their alt leaders screen. For example, Scotland could consist of Robert the Bruce, James VI, and then Victoria, George V, and Churchill. Alongside that, England contains Henry III, Elizabeth, Victoria, George V, and Churchill as well.