HARTLEPOOL, England — Britons are no longer living longer than before. Just ask Callum Hills.

His father died of a heart attack last year at age 52. Mr. Hills had found him stricken on the floor in the middle of the night, and today is still haunted by his memory.

“I keep having dreams, and he’s in them,” said Mr. Hills, a thoughtful and articulate 23-year-old.

While attention is riveted on the Brexit turmoil in Westminster, with dire forecasts of possible chaos, food shortages and recession, a dispiriting trend is already visible in struggling towns and cities across the nation.

For the first time in modern history, Britain’s gains in life expectancy have stalled — at 79.2 years for men and 82.9 years for women for the years 2015 to 2017. That is better than the United States, but Britain is slipping down the ranks in Western Europe.

In 1841, life expectancy in Britain was around 40 for men and 42 for women, and its rise through improvements in infant mortality, sanitation, nutrition and medical care became a symbol of progress. In recent times, the figures had generally improved by about three years every decade.