Without Wales the British aren't British. Which is actually the sloagan used to stop the independance movement in Wales. The Britons were dislocated by the Anglo-saxon invasions and ended up in wales.
Scotland controls the throne, since James VI of Scotland became James I of England the monarchy has been united. They would have to give up their Monarch to be independent, and without the monarchy chaos alone reigns.
Beyond that, both countries still need briton for security and economic prosparity. England remains a Great Power without Scotland and Wales, but Scotland and Wales become bit players in Europe and the World.