There is a common misconception that London is always foggy. I'm not sure where this comes from, possibly from Hollywood movies of yesteryear that depicted London's streets this way to make them more sinister. I don't know.
The reality is I have rarely seen fog on London's streets, yesterday was one of those rare occasions. This picture was taken looking East from the Millennium Bridge yesterday at around 9AM. Usually you can see a few miles without much difficultly here you can only see a few hundred meters. It's probably how it will look when "Another American Warewolf in London" is made :-).