Prince Edmund claims the title of Duke of Edinburgh in 1485, but this title was not created until 1726, just 19 years after the final union of England and Scotland. This is one of many gags in the series, as explained on the DVD special features. See more
It will be a great day tomorrow for we nobles.
Well, not if we lose, Percy. If we lose, I'll be chopped to pieces. My arms will end up at Essex, my torso in Norfolk, and my genitalia stuck up in a tree somewhere in Rutland.