Celtic nationalism. Get more appreciation for the Gauls (that survived Caesar) were Celtic in origin, so you can get a ring of along the outer portion of the state with some feeling of solidarity for each other. Later on a larger amount of Nordmenn becoming Norseman might help to somehow tie together the Angles, Saxons, Jutes, Franks, and Goths that had taken over portions of the land over the preceding centuries. Maybe going far back enough with Roman colonies can help? I know, really out there. Having a greater connection between northern France and southern England might help in this manner, though having the ruling and colonizing classes around might not do much more than drive out or keep down the locals. Which, without support from Italia, would be unable to keep down hoardes of Celts fighting for their lives and self-respect.
France and England wouldn't exist in quite the manner we think of it now, though. Besides, There used to be gavel kind in both areas. Made sense given distances and how hard it would be to govern them effectively. Maybe have all of Henry II of England's sons survive? I forget how the Avengin Empire was planned to be divied by I believe England went to one, Aquitine to another, Normandy and Brittany to one more, Scotland being given to the fourth (maybe through marriage?) while Ireland was partially conquered so the youngest, John, could have land of his own. Having this dynasty helping each other out, maybe going on crusades with each other, marrying into each other occasionally, could allow them to be politically tied together, even if it ends up as it did IOTL with fights by cousins in England and France (though the Royal areas of France {With the central 'French'} were outside this land. I imagine they could expand.} over the inheritance some generations down the line. I can see this area being a decent economic unit, as outside of spices they would have food, wine, lumber, various metals, wool, grazing land, and all that to trade between each other. Plus, besides for Scotland and Ireland, the three portions would have decent incomes, plus the status needed to marry into the highest levels of royalty, even if the Duke of Aquitine wouldn't have a kingly crown. Though maybe Aquitine went to the King of England with Brittainy and Normandy going to two different ones? I forget.