Here is the big economic question for the U.K.’s Brexit negotiators when they eventually get around to talking about the future: To what extent will British companies be free to sell goods and services inside the European Union after Brexit without confronting a plethora of new bureaucratic hurdles?

An easy way to avoid them would be for the U.K. to remain attached to the EU’s single market and customs union. But there is a price: London would have to follow EU regulations, submit to the jurisdiction of the European Court...