No one denies that clean air is important but this will make negligible difference in the outer boroughs. This was the result of Sadiq Khan’s own report. The ULEZ clean air claims are just not substantiated.
Many of the figures are debateable and massaged to fit the narrative. Outer London boroughs generally have good air quality. The higher numbers are from a small sample at particular times of the day, e.g. outside schools. This can be managed in other ways.
A compliant car with borderline emissions can drive around all day for free, whereas a car just over the limit might only spend a few minutes a day in the zone. Who is the bigger polluter?