The Breakup of Britain”? It sounds like a fantasy fiction title. To many people across the world, including the English themselves, it is inconceivable that this deep-rooted United Kingdom, the oldest royal democracy in the world, could split apart.

In the last few weeks, however, official London has panicked over the rising clamor of voices from all over the British Isles suddenly agreeing that the archaic structure of “Great Britain” is overdue for a shake-up — even a breakup.

Nowhere are these voices in better harmony than in Scotland. If “Britain” is more than a word on a passport, why do most Scots now feel their primary identity is not British? What would it mean to be English if the Scots walked away? And should the Welsh follow them? A fresh wind of new ideas is blowing from Scotland and tempting all the queen’s subjects to reimagine their identities.

Last month Alex Salmond, Scotland’s first minister and the leader of the Scottish National Party, introduced a “consultation document” on a referendum to decide his nation’s future. After an unexpected triumph in last year’s elections to the Scottish Parliament, the party is now fulfilling its promise: a vote to declare independence.

If Mr. Salmond has his way, the vote will take place in 2014, just shy of 700 years after King Robert the Bruce defeated the English at Bannockburn. And he wants only one question on the ballot paper: “Do you agree that Scotland should be an independent country?”....