Possible Clues:

• The Channel Islands were once part of the Duchy of ____
• Historic duchy, as standard, has a Duke
• Where William came from, and not Amanda
• Partial coincidence of two girls found somewhere in north-western Europe
• Operation Overlord locale
• Part of France
• Standard hotel not found in accessible old province
• Two chaps getting together somewhere in France
• City incorporating weirdly random part of France
• Where the D-Day landings took place