I wondered at the substitution of London for Nantes too -- especially as in the old city of Nantes, the Loire used to run right past the chateau des ducs de Bretagne (chateau de la duchesse Anne), feeding an encircling moat. The navigable river has largely receded and the "moat" is now purely decorative, but at one time an inmate of the "Vieux Donjon" (a modern restaurant in one of the towers reflects its earlier use) could indeed have leapt into the moat and perhaps swum across the Loire to freedom. An impossible leap today. But a staple of French folksong, and in France, at least, it's always NANTES where the prisoner is held.   

So thanks for the suggestive info on Charles d'Orleans! His son married Anne de Bretagne,
who brought her duchy of Brittany, as dowry, into the kingdom of France. (And to this day, I'm told, as a condition of that dowry, highway tolls stop at the border of Brittany.) It sounds like the Arcadian version of the song has more behind it than simply confusion of sound. . .