A senior aide to British Prime Minister Boris Johnson has quit over a leaked video clip showing her and other officials joking about a Downing Street Christmas party during last year’s COVID-19 lockdown. Johnson and other government ministers had repeatedly denied claims, first reported by the Daily Mirror newspaper a week ago, that Downing Street staff held a party on Dec. 18, 2020, in violation of lockdown rules. But leaked footage obtained by ITV showed former press secretary Allegra Stratton laughing with other Downing Street staffers as she appeared to rehearse answers to questions over a lockdown-busting Christmas party. On Wednesday afternoon, Stratton resigned from her role as the prime minister’s spokeswoman for the COP26 climate summit, and apologised for her remarks in the footage. In a tearful resignation statement outside her home, Stratton said: “My remarks seemed to make light of the rules, rules that people were doing everything …