FFORESTFACH, Wales — In a 2016 referendum, Stephanie Holtom voted to leave the European Union, worried about immigration and convinced that other countries were telling the British government what to do.

But outside a supermarket recently in a large, suburban strip mall not far from the Welsh city of Swansea, Ms. Holtom conceded she might have been wrong.

“I agreed to come out of Europe, but I am beginning to have second thoughts. I think it’s a mess, and I’m sick to death of it,” said Ms. Holtom, who is retired, as she collected her shopping cart. She added that, if there were a second referendum, “people would vote to stay.”

Since a majority of Britons voted narrowly to leave the bloc more than 18 months ago, most politicians have treated a withdrawal, known as Brexit, as inviolable. Even amid signs of a slowing economy, few saw signs of a shift in public opinion.