While watching 28 Days later at the beginning of the movie, the main character is walking through the desolate streets of London.
If you watch carefully, you can spot a person from far away walking down the streets toward the camera! This occurs briefly at the 11 minute mark of the movie (after credits and such)
Given that we know there are other people in London (we meet another four during the film) and that the infected are everywhere, I don't see that is a slip-up at all.