Continuity mistake: When Jim is wandering the deserted streets after waking up from his coma, there is one overhead shot of Jim walking down the street, towards a silver car. When it then cuts to a closer shot a red double decker bus appears underneath the "Landmark Advertising" billboard. A small white van/truck has appeared in front of a medium-sized white lorry, and the position of the silver car parked in the centre of the traffic lights grid changes between subsequent shots. Also between these and following shots, the position of the trash littering the streets differs. (00:11:05)
Chosen answer: According to the making of documentary, they cleared relatively small portions of London early in the morning (with the help of the Police of course) and filmed the shots using a digital camera, which is incredibly easy and fast to set up, meaning they could go in and do a take in a remarkably short time. So it is the real London (as well as the motorway scenes), but only small portions at a time are deserted.
Gary O'Reilly