One important aspect of Britain’s possible exit from the European Union hasn’t been much discussed. If its people vote to leave the EU in a referendum to be held in all likelihood next year, how would the U.K. extricate itself from its 40-year membership in the bloc?

The answer is with extreme difficulty. Officials and experts say the process of adjusting to existence outside the EU would likely consume the British Parliament for years, making it unlikely the government could focus on much else.

“This...