LONDON—For many of the thousands of Russians who have helped lend this city the nickname Moscow-on-Thames, the nerve-agent poisoning of one of their countrymen is a jarring intrusion from a world they thought they left behind.

They have little in common with the older waves of emigres who have earned the ire of the Kremlin, the wealthy oligarchs lured in part by a controversial investor program, or the likes of Sergei Skripal, the 66-year-old Russian double agent now fighting for his life.

Instead, they are young, highly educated professionals who left Russia for economic, not political reasons.

“These people didn’t run away from a bloody regime,” said Anna Chernova, a project manager at a Russian-language magazine that hosted a party last week for Russian yuppies in London’s hip Shoreditch neighborhood. “They are here because of opportunities.”

On Monday, Prime Minister Theresa May said it was highly likely that Russia was responsible for Mr. Skripal’s poisoning. She gave Moscow until Tuesday to respond, warning of a tough response if the answer was inadequate.