2006 United Kingdom general election

By Christmas 2006, Harriet Jones had become Prime Minister of the United Kingdom after winning a landslide majority in an election. (TV: The Christmas Invasion)

Background
By March 2006, the British government had been infiltrated by the Raxacoricofallapatorian Slitheen family, whom murdered and disguised themselves as government office. By fabricating evidence of alien threat, their aim was to gain access to nuclear weapons which was to be used to start a nuclear war, reducing the planet Earth to molten slag which they intended to sell for profit.

As they staged the crash landing of an alien spacecraft in central London, the Slitheen killed the British Prime Minister, whose body they hid within 10 Downing Street. The position of Acting Prime Minister was assumed by MP "Joseph Green", actually Jocrassa Fel-Fotch Passameer-Day Slitheen. (TV: Aliens of London)

Ultimately, the Slitheen plot was thwarted by the Ninth Doctor and Rose Tyler with the assistance of MP Harriet Jones, a backbencher serving the constituency of Flydale North, who found the Prime Minister's body with Rose.

After 10 Downing Street and the Slitheen inside were destroyed by a harpoon missile, the Doctor, who recognised Harriet's name, witnessed her taking charge in the aftermath, explaining to Rose that she was destined to become Prime Minister, elected for three successive terms, and the architect of Britain's Golden Age. (TV: World War Three)