Kenneth Clarke

Kenneth Clarke was a Prime Minister of the United Kingdom. According to the Eighth Doctor, he immediately succeeded Tony Blair. (PROSE: Interference - Book One)

Behind the scenes
Clarke's forename wasn't given in Interference, but the common-sense interpretation of the passage is that the author was referencing the real-life Kenneth Clarke. (However, it's also possible that the reference was to .)

In real life, Clarke never became Prime Minister or Conservative Party leader.

If Clarke really did follow Blair in the DWU, he'd logically have had to have been the party leader instead of and won the 2005 election. If that's the case, then he was the Prime Minister who was murdered by the Slitheen in 2006. (TV: Aliens of London)