A consensus seems to have emerged in the past week that the U.K. is heading for the softest of Brexits. Lawmakers and government officials appear convinced that Prime Minister Theresa May’s proposal of a U.K.-wide customs arrangement as the backstop solution to the Northern Irish border problem and her government’s apparent agreement to allow Parliament some form of meaningful vote on the final Brexit deal have effectively removed the prospect of a chaotic “no deal” crash out of the European Union.

Even hard-line Brexiters...