Just thought of one. England and Scotland. Now before you throw rotten fruits or vegetables, hear me out. What if Scotland and England had been unified earlier on with better terms? Doesn't even need to be all of it. Perhaps just the Lowlands, with the Highlands ending up as a sort of Wales.
The obvious POD is having poor https://en.wikipedia.org/wiki/Margaret,_Maid_of_Norway, survive, and marry as planned, Edward of Carnarvon, uniting the English and Scottish crowns. At this time, many of the greater Scottish lords also have major land holdings in England, so there are incentives to make this work.
Of course, given what an ass Edward I was and what a berk Edward II turned out to be, the English could very well screw this up.