The opening sequence (set pre-2000) contains a shot of London, including the Houses of Parliament, with a black-roofed building opposite (which, in fact, was not built until 2000). Later during the movie, now set in the present day, a similar shot correctly shows this building under construction, with bare foundations.
Van Helsing's office building is in London, but the Security Guards on duty are armed. Security guards in England do not, and cannot, carry guns. Handguns are illegal in the UK and private citizens, even security guards, cannot even possess handguns, much less can they carry them about. Handguns can only be carried by the armed forces and by specially trained police officers (and even then it is rare to see even an armed police officer).
(at around 1h 8 mins) In the long hallway scene at Mary's house, the female vampires are crawling on the hallway walls. Mary has her back against the wall, however, its obvious that the scene is filmed with Mary lying down and the camera rotated for the effect. As she turns her head her hair falls backwards, towards the wall.