LONDON — In Scotland there were boos, in Wales threats of unrest, and on Wednesday, things went little better in Northern Ireland for Britain’s new prime minister, Boris Johnson, as he wound up his tour this week of the nations of the United Kingdom.

If Mr. Johnson’s roadshow around what he calls the “awesome foursome” was intended to convert his critics, it instead underscored the hostility in many corners to his promise to lead Britain out of the European Union, or Brexit, on Oct. 31, without any agreement if necessary.

That pledge has already alarmed currency traders and sent the pound sterling tumbling. A morning of breakfast talks in Belfast provided the latest signs of problems for Mr. Johnson, who has added “minister for the union” to his job title.