Camilla, Duchess of Cornwall, accompanied by the Dean of Westminster, walks by the tomb of the Unknown Warrior during a candlelit vigil and prayer service at Westminster Abbey in London on August 4, 2014, the 100th anniversary of Britain's involvement in the First World War. (Photos by Sang Tan – WPA Pool /Getty Images)
Westminster Abbey's website explains:
Drawing upon Sir Edward Grey's famous remark that "the lights are going out all over Europe", Westminster Abbey will mark the centenary by moving from light into darkness, until one candle remains at the Grave of the Unknown Warrior.
The Duchess of Cornwall, representing the Queen, extinguished the final flame at 11 PM to mark the moment of the declaration of war.