LONDON — If you ask British voters what sort of plan for leaving the European Union they support, you tend to get hesitant, vague answers.

But ask them what they oppose and you hear forceful clarity. No to Prime Minister Theresa May’s deal. No to leaving the European Union without a deal. No to “remoaners,” as tabloids call those who want to stay in the bloc.

No to Ms. May herself, whose approval ratings are deeply negative. No to her rival and leader of the Labour party, Jeremy Corbyn, whose poll numbers are even worse.

In a recent YouGov poll asking Britons whether Ms. May or Mr. Corbyn would make the better prime minister, the runaway winner was “not sure.”