Forty-eight hours before President Biden and former President Donald J. Trump clashed onstage in Atlanta on Thursday, the leaders of Britain’s two major parties, Prime Minister Rishi Sunak and Keir Starmer, went head-to-head in Nottingham, England.

To say their debates were different doesn’t begin to capture the Atlantic Ocean-sized chasm that separated them.

In content, tone and atmosphere, the British debate showcased two politicians in their prime, sparring over the issues — frequently heated, not without personal jabs, but focused on the policy nuances of taxes, immigration and health care. Neither Mr. Sunak, 44, nor Mr. Starmer, 61, brought up his golf handicap.

Britain and the United States are often viewed as operating under the same political weather system — the conservative turn to Margaret Thatcher and Ronald Reagan, the pivot to youth and the center-left with Bill Clinton and Tony Blair, and the anti-establishment, populist backlash that fueled Brexit and Mr. Trump. But this week’s back-to-back encounters showed how sharply these democracies have diverged, at least in this election cycle.

“These are two countries in very different places, with very different views of their place in the world,” said Kim Darroch, who served as Britain’s ambassador to Washington during the Trump administration.

“The tone between Sunak and Starmer was that of two profoundly earnest politicians,” Mr. Darroch continued. “Between Biden and Trump, it was barbed, it was nasty, it was childish, but it was not earnest.”