LONDON — Just after work on Hallowee n, people congregated in a cavernous steampunk-themed bar in the financial district here, drinking No Deal Daiquiris, Bo-jitos and Tequila Mays: cocktails dreamed up for an “End of the World Brexit” party, planned for a night when Brexit was supposed to happen but didn’t.

A string of European Union flags hung above the bar. An actor dressed to look like Boris Johnson was roving, posing for selfies and pontificating about Brexit in character to anyone who would listen. (The actor, Will Barton, has played Mr. Johnson onstage and said he makes occasional appearances like this as a side hustle.)

Damien Rivoire, 41, was wearing a mask and carrying a sign that said, “Ditch Boris!!” — a reference to the prime minister’s declaration that he would rather be “dead in a ditch” than ask the European Union for another delay on Brexit.

“It’s a scary time,” said Mr. Rivoire, a photographer who is French but has lived in London for 14 years. “I knew Boris would be here tonight and I thought this was a good message, since Brexit didn’t happen.”