If the Royals in Spain can dig up an old dusty law then you can be sure the British Royals have the same kind of loophole they can use at the last minute. Letting it appear like it is a way to gain freedom would inspire others to try the same thing and the uprising would be deemed to be treason against the Crown so the rebels can be executed.
Running to Brussels is where the owners of the Royals live so one has to wonder if the Leader folded a bit too easy. It is no different that Lebanon having a Leader that runs to Saudi to take care of his billion dollar business as well using it as a hideout when he fails to get elected. Lebanon under Saudi control makes the offshore resources more or less an Israeli resource.
He's run to Brussels because it's the capital city of the EU. He's trying to get EU help for his cause.