Mjit, the problem with London is that they have become the centre of the country so attract the largest population, service industries, etc hence high use of personnel transport. I do believe the most capital per head for public transport is spent in London than anywhere in the country which does not seem to work in reducing your pollution at present.
HS2 confirmed the idea that everything revolved around London, as the project always reference reduced times to get to London.
An answer would be to decentralize of what goes on in London, for example my son had to drive from Nottingham to Heathrow so his girl friend could catch a flight to see her parents, so you have to pollution of the car trip and going into London. Train transport in Nottingham is poor, talk of electrification (East coast line) has been on the books for decades, but each government spends elsewhere. HS2 money has not been allocated for this and now suggesting we will have to wait until 2030.
If the government developed a better transport systems that served the population and not feed London then pollution in London may drop.
The only true solution to reducing pollution in London is to stop the use of all fossil fuel transport full stop, only too happy for Londoners to do this.
Graham