There you go with your ancient history again.
Try to catch up with the present.
It is what people want now that counts.
If, years ago, any 6 counties such as Jim mentioned ended up being governed by France, and if the people of those counties wanted to stay French, only a fascist would put a gun to their head and force them back to British rule.
Scotland only has to vote for independence and it will get it.
Likewise Wales, Cornwall or any other part of Britain.
Never mind what happened before any of us alive now were born.
Let people choose their future if they cannot change the past.

And, believe me, no one will be more overjoyed than the people of mainland Britain if you ever manage to persuade 51% to vote to leave!
As soon as you like.
Please.