The actress is famed for her bohemian views, and her own great-grandfather was the liberal Prime Minister H. H. Asquith, and so her longtime friendship with the Conservative leader has shocked many fans.
The actress, who was photographed spending New Year's Day (01Jan11) with Cameron and his wife Samantha, insists her politician pal is far more liberal than the British public thinks.
She tells Style magazine, "He's not that conservative, actually. I mean, he's not a right-wing person. If he was in America, he'd be a Democrat, and he's got a hilarious sense of humour, which nobody really knows about.
"(I was) amazed (he became Prime Minister). Not in a bad way, but it's very strange when somebody your age becomes a (leader)."