If you mean you don't want any of the duchy titles for England created at the start of the game, that's easily possible.
If you mean you don't want it to be possible to create duchy titles for England at the start of the game (and only after some sort of event or decision by the King of England) then things get problematic. How is this implemented mechanically? What would be the requirements for creating the rank of duke? What happens if the King of France conquers Cornwall and wants to make somebody duke of it?
If you mean you don't want it to be possible to create duchy titles for England at the start of the game (and only after some sort of event or decision by the King of England) then things get problematic. How is this implemented mechanically? What would be the requirements for creating the rank of duke? What happens if the King of France conquers Cornwall and wants to make somebody duke of it?