9. Margaret Thatcher
This would be one for a few years time, I expect, seeing as the first female prime minister's death is still fresh in people's memories. The Doctor's already met Winston Churchill, so why not Margaret Thatcher? He could even be going to visit Churchill and ending up in the complete wrong year! I can imagine him calling her Maggie or Mrs T, her getting annoyed, and then needing him to save the world. I don't know when 10 Downing Street's bulletproof shields were installed (or even if they exist in reality) but he could either use them, because of his knowledge about them from 'World War Three', or invent them, because he knows they will be useful in the future. Maybe he arrived at that famous terrace thinking they were visiting David Cameron and then end up in the wrong year. Obviously, as I mentioned, she has only just died, but Thatcher is an iconic British figure and one that we should be proud of, and show off to the world. Where better than on a TV show famous across the continents? She's certainly strong enough to take on the aliens and enemies the Doctor would throw at her.