Mr. Saxon

Mr Saxon was a politician on Earth. He was active from at least April 2007 (DW: Love & Monsters) and was elected Prime Minister in 2008. (DW: The Sound of Drums / Last of the Time Lords)

Behind the Scenes
"Mr. Saxon" was the arc word for the late part of series 2 and all of series 3 of Doctor Who, just like "Torchwood" and "Bad Wolf" had been for series 2 and 1. The phrase also featured in Torchwood series 1.

It has been theorised that Mr. Saxon is actually the renegade Master, hence the episode title "Last of the Time Lords" and why he is to be an enemy.