LONDON, July 4 -- After 319 years of political union, three of the United Kingdom's four nations are now governed by parties whose founding purpose is to end it. The planning for what comes next, once theoretical, has become operational.

The Guardian reported on Saturday that political leaders across Scotland and Wales have begun quietly mapping the constitutional steps that would follow a Reform UK victory at the next Westminster election, a scenario that current polling places as more likely than not by 2029. The plans are not identical, and their architects are careful to present them as contingency rather than ambition. But they exist, which is itself new.

The Scottish National Party, having won its fifth consecutive Holyrood electi...