Every so often, Britons in large numbers seem to rally around a common theme, a newly coined orthodoxy that brooks no dissent and rudely demands change. And, as events over the years — indeed the centuries — have shown, their leaders ignore such junctions in the nation’s history at their peril.

It happened, perhaps mawkishly, when Princess Diana died in 1997 and the royal family — perceived as stuffy and unresponsive, remote and unfeeling — seemed to disdain its subjects’ communal grief. Only when Queen Elizabeth II addressed the nation, descending from her palace to street level to bow — however discreetly — at Diana’s funeral cortege, was the monarchy redeemed.

It happened again in 2003 when over a million people took to the streets of London to tell the then-prime minister, Tony Blair, that they did not want their soldiers to fight in the American-led invasion of Iraq. But Mr. Blair did send his troops to war and became embroiled in the acrimonious debates over Iraq’s purported weapons of mass destruction that undermined his every justification for doing so.