"Monarchy and the End of Empire" by Philip Murray, head of the Institute of Commonwealth Studies, quotes a diary kept by Louis, Earl Mountbatten, uncle of Prince Philip and former viceroy of India. The queen celebrated her Silver Jubilee in 1977.
In his diary, Mountbatten said he asked the queen why she looked "cross and worried." She was apparently concerned that Amin, who had not been invited with other Commonwealth heads of state to attend the Silver Jubilee ceremonies, would show up at a service at St. Paul's Cathedral.
"She laughed and said, 'I was just thinking how awful it would be if Amin were to gatecrash the party and arrive after all'," Mountbatten wrote. "I asked her what she had proposed to do and she said she had decided she would use the City's Pearl Sword which the Lord Mayor had placed in front of her to hit him hard over the head with."
Murray suggests Mountbatten's diary be taken with a "pinch of salt." But in his book, he suggests that the queen, while her duties in Britain were largely ceremonial, played an important role at a time when many Commonwealth countries had dictatorial regimes or were riven by civil strife.
The book said the government had plans in place in 1977 to stop Amin if he tried to enter Britain. Amin was deposed in 1979 and died in 2003 after years of exile in Saudi Arabia.