Was there some philosophical development in the idea of nations or nation-states during that time? It appears that Queen Matilda was the first person to have an “England” title. It also appears that the Duke of the Normans -> Duke of Normandy switch happened at the same time.
Source? I'm honestly quite curious. Is there a website that lists their titles? Are you looking at documents/manuscripts?
If I may add a follow up question: does anyone know if the reasons behind this change are similar to the change from "Roi des Francs" (king of the Franks) to "Roi de France (King of France) during the XIIe century in France?