Fewer than 6,000 people live in Trevethin and Penygarn, a gritty, deindustrialized district amid the hills and valleys of South Wales, and not many expected drama when a vote was called to elect a member of the local municipality, one of the lowest tiers of British government.

But as one candidate, Stuart Keyte, a member of the populist anti-immigration party Reform U.K., campaigned outside Trevethin’s small supermarket on Wednesday, a volley of eggs rained down on him from behind a van parked nearby.

Sidestepping quickly, Mr. Keyte, who happens to be a former member of Britain’s elite Parachute Regiment, avoided the worst of the barrage from the unknown assailant, then drew on his military experience. “The closer you get to the target, the more flak you get,” he said as he inspected the egg yolk splattered on his shoes.

He hit his target on Thursday, when he became the first member of Reform U.K., the upstart party led by Nigel Farage, the Brexit campaigner and ally of President Trump, to win an election in Wales.

Turnout was low, as it usually is in municipal elections: just 973 people (less than a quarter of those eligible) voted in the contest for a council seat in Torfaen (pronounced Tor-vai-uhn) municipality, which remains under Labour control and includes Trevethin and Penygarn. Mr. Keyte won 457 votes.