I enjoyed that movie as well. The actress you’re thinking of is Sophia Myles. Since she was naked in most of her previous film, she was an interesting choice for this one. It’s a pity that she hasn’t done anything other than British television.
She starred in the American series, "Moonlight" with Alex O'Loughlin. He's a vampire who is also an investigator; she's his sidekick/eventual girlfriend who works with him on cases. They had great chemistry.