The prospect of an orderly Brexit process in March 2019 looks as remote as ever. The European Council of state leaders appears to have rejected Britain’s “Chequers” proposal and set a deadline of October 18 to reach an agreement. Without that there will not be time to approve withdrawal legislation to secure customs arrangements before the U.K. leaves the EU. That would put regional supply chains at risk of significant disruption. Manufacturers are making contingency plans. BMW, for example, will pause production at its Mini factory in the U.K. during the month following Brexit, which...