In the coming months, Britain will make a momentous decision: whether to stay in the European Union or leave and go it alone. The outcome will have a long and lasting impact, on the jobs and living standards of British citizens, on the fortunes of many global companies, and on the U.K.’s influence across the world.

Similar concerns were at play more than a decade ago when the U.K. decided to stay out of the single European currency. Not adopting the euro was the right decision then. Staying in the EU would be the right decision...