To be honest a city like London could do with banning non-commercial / public transport traffic entirely, with an exception for blue badge holders also. Replacing it with efficient and free/cheap public transport for all.
With that being unrealistic, though, I think 2006 is a little bit harsh? There are plenty of pre-2006 cars that are hardly spewing out emissions. It should be based purely on the emissions output of the car. A 2005 focus is quite obviously producing a lot less pollution than a 2007 transit
