Without mentioning Hillary Clinton or Donald Trump, the ambassador then assured the crowd that ahead of the presidential election in November, candidates would need wider appeal.
"In all the Democratic and Republican primaries so far," he pointed out, "maybe 12 or 15 per cent of the electorate has participated. You can win a nomination appealing to small groups of people. When the presidential election comes around, you have to appeal to the whole country."
Asked to vote on whether the US was doing enough to solve the world's problems, 59 per cent of the audience said it was not. Around one quarter said the country was "too involved".
Mr Barzun had previously run the poll at more than 100 sessions nationwide. "If you add them all up, answers from all over Britain, two-thirds think we're too involved around the world - but it flips here with this group."
More than half the audience said Middle East peace was more important than concerns over Daesh, Britain's role in Europe, and climate change.
