Joseph Green

Joseph Green was a politician and chairman of a Parliamentary committee for the monitoring of sugar standards in exported confectionary. He was slain by Jocrassa Fel-Fotch Passameer-Day Slitheen, and his skin then used by the Slitheen as a disguise to become the Prime Minister of the United Kingdom and cause World War III. Green was the highest-ranking member of the government within London during the time of the invasion and then became Acting PM; Jocrassa Slitheen cancelled airlifts into London for the Cabinet of the United Kingdom in order to keep this position. (TV: Aliens of London)

Behind the scenes
As a committee chairman, Green could not have been the highest-ranking member of government - he wouldn't have been a minister.