Five months after his Labour Party won a thumping election victory, Prime Minister Keir Starmer of Britain finds himself a lonely figure: Like-minded centrist leaders in France and Germany are in retreat, while in the United States, Donald J. Trump and his populist message have vanquished the Democratic Party.

On Thursday, Mr. Starmer served notice that he would stick to the plan for rebuilding Britain that won him the election, though he did put more emphasis on kitchen-table concerns, promising to boost voters’ disposable income, construct 1.5 million new houses, and put more police on the streets.

Mr. Starmer’s six goals — delivered in a speech that was yet another effort to reset his troubled government — amounted to a stubborn bet: That he can resist the populist wave rolling across Western democracies by delivering on the issues voters care about.

“Everyone can see there’s a growing impatience with traditional politics,” Mr. Starmer said. But he added, “Populism isn’t the answer to Britain’s challenges. Easy answers won’t make our country strong.”

For Mr. Starmer, a methodical lawyer-turned-politician, the speech was a clear admission that his government has continued to struggle — a string of missteps and minor scandals sapping its poll ratings and giving an opening to populist critics like Nigel Farage, a close ally of Mr. Trump.