What lost France to England was the end of the French Civil War. The 100 years War is in fact the factor which created a separation between English and French citizenship (even if the word itself will come later). The final reconquest by the French King of Normandy, Brittany and Burgundy went without one city closing its doors to the French armies. The now French had enough of now English occupation. Somehow, the nationalities became before the former feudal link. So, in order to keep France as part of a United Kingdom, the English King would have to stop both side to think to be enemies.
Even today, reading and believing some free-newspapers readers comments, it is still not achieved...
Even Hasting (1066) is described as a battle of the English King against the Normans (so France did not exist but England did apparently) as the English refuse any notion of being defeated by French.