Many businesses “are going to think very hard before choosing the U.K. if they don’t know whether it will be part of the integrated market” of the European Union, said Thomas Sampson, a lecturer at the London School of Economics. Airbus’s decision to publicize its stance, Mr. Sampson said, suggested that it didn’t feel the government was listening to its private appeals. “It’s another signal,” he continued, “of how unhappy the business community is.”