> Why not further devolve sovereignty further to the british isles, regions, cities or counties?

That's a great idea, and precisely what they do in Switzerland. Stuff that's local is decided locally. What days are holidays? Depends on what canton you're in. How are schools organised? Local. What are taxes? Local.

That doesn't mean you can't organise some things on a national level, for instance the military, and federal projects like the excellent technical universities. And there's a federal tax to go with it.

If little regions had more power it would be a lot easier as an individual to shop around for the area you like, and for experiments to be made.