You can set it it as a province but it's not as simple as, making africa playable in the Medieval campaign map. Africa was just a disabled province wheras Frankland wasn't designated as a province to start with. It can be done though.

This would be an interesting mod, but you'd need a small Roman presence, as well as the many different British factions and of course the Angle, Saxon and Jute invaders.

I believe that the current British isles map would not be sufficent though, it would need to extend further eastward.