There are only 50 British actors, and they trade off roles in movies and TV. There's ridiculous overlap between Doctor Who, Torchwood, Spaced, Life on Mars, and even Ashes to Ashes (only three episodes in and already I've spotted an actress who was on both DW and Spaced).

Hugh Grant was up for the Gilderoy Lockhart role, by the way, so that counts as his Harry Potter connection.