theres a churn of car ownership which has a natural beat to it. brand new cars get passed down after 3-4 years into the 2nd hand market and so on, and the proportion of new cars is now significantly more clean - either through new engines being far more efficient or through complete moves to BEV. So the process of becoming cleaner over a decade is well under way. These old cars may have been moved out of london, where the pollution is worst because of all the factors (oxford streets’ strange ecosystem airflow for example because of the height of the stone building, creates a strange envelope which traps pollution in and circulates it) but they will soon be replaced by cleaner and greener cars regardless of where they were moved from because the overall mix of engine type is rapidly changing. Overall, the same filthy 2006 vehicle is less polluting in a different setting, with a different use case and if its older than 2006 probably about to have its wheels fall off in any case.