Asquith Slitheen

A member of the Slitheen family posed as Oliver Charles and then General Asquith, using their skin at different points as skin suits. (TV: Aliens of London) In 2006, he, along with other members of his family, intended to turn the Earth into a radioactive wasteland to sell as cheap starship fuel as part of the "family business". (TV: World War Three)

After becoming Asquith, the Slitheen said it was "a shame" as he "quite enjoyed being Oliver. He had a wife, a mistress, and a young farmer." The Slitheen revealed himself in front of a group of alien experts from UNIT inside 10 Downing Street shortly before they were electrocuted by the ID badges they were wearing. (TV: Aliens of London)

The Ninth Doctor survived the attack and placed the ID badge on this Slitheen, causing the Slitheen and the rest of his family members to be electrocuted. After the upper levels of Downing Street were quarantined, he joined Jocrassa and Blon in their hunt for the Doctor, Rose Tyler and Harriet Jones. When they sealed themselves inside the Cabinet Room, he continued with his plan.

Before the United Kingdom's nuclear launch codes were granted to them by the United Nations, Captain Price warned the Slitheen of an incoming missile. Unable to get back into his skin suit of General Asquith in time, this Slitheen was killed when the missile hit. (TV: World War Three)

Behind the scenes
The implication in Aliens of London of this Slitheen "enjoying" being Oliver Charles was confirmed by Doctor Who: The Encyclopedia, which stated that the disguised Slitheen had a number of sexual liaisons with Charles' wife, mistress, and young farmer.