That flag has nothing to do with the Gunpowder Plot.

The flag of England and Scotland under James in 1605 was this
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSgKWwOe46ejFiEFU1rL6fCUy0NrBiZ1mZREgO_tPVSbO9Bue3AqbKpPxY&s

The Catholic terrorists under Catesby didn't have a flag