For new U.K. Prime Minister Boris Johnson, Brexit isn’t about Britain turning its back on the world. Quite the opposite: leaving the European Union is a chance for a sovereign Britain to stake out a new, more free position in the global economy.

Yet if there was ever a bad time for a lone economy to put itself at the mercy of the world, this is it. The global economy has changed in the three years since Britons, with Mr. Johnson’s encouragement, voted to leave the EU. The same sentiments that drove that vote have manifested...