That’s because drivers are avoiding entering the ULEZ in the south of London and are staying south of the South Circular Road.
I know London very well having been born there and lived there for quite a while, my wife even longer. My relatives that used to live inside the M25 are no longer with us and what few I have left have all moved outside. My parents are both buried inside and if I ever feel like visiting their grave I shall have to pay the punitive fine.
None of this will make any difference to the air in London.