Backbencher
From Tardis Wiki, the free Doctor Who reference
A backbencher was a Member of Parliament, or MP, who was a junior member or for some other reason did not hold government office and was not a spokesperson for the current opposition.
MP Harriet Jones, of Flydale North, described herself as a backbencher. (TV: Aliens of London) Heathcliffe Bower was a conservative backbencher during Margaret Thatcher's time in office. (PROSE: The Assassin's Story)