Hawthorne (The Beast Below)

Hawthorne was in charge of the Government of the Starship UK, put in charge under The Queen at the time. He was responsible for making the Smilers, a race of androids that were used to police the Starship UK, and turning the space station into a police state; he believed this was necessary for the continuation of the Starship. It was he who directed the Star Whale through torture.

While he initially seemed to have usurped control from the Queen, he was revealed to have been acting under her original orders in regards to the Whale: every time she came close to the truth, he would take her to the Tower of London and reveal the whole truth to her, giving her a choice about whether to wipe her mind or release the whale. The Eleventh Doctor uncovered the situation and Amy Pond found out the Whale would direct Starship UK without torture, a fact that stunned Hawthorne. What happened to him after this is unknown. (DW: The Beast Below)

While his position was never stated on-screen, he could - as he heads a British government - be the British Prime Minister of the time.