In 2015 it was estimated that there are around 10,000 deaths a year in London due to poor air quality. In the UK as a whole the estimate is 40,000 deaths which compares to around 8,000 due to alcohol and around 100,000 due to smoking. Many more suffer from respiratory illnesses and most of this pollution comes from particulates and NO2 from diesel vehicles.
The problem is worse now because for the last 15 years the UK government successfully encouraged a switch from petrol to diesel cars in order to lower UK CO2 emissions. While some will have been aware of the likely health effects of this policy from the outset there may have been optimism that those would soon be lessened by a switch to cleaner diesel engines that had to pass ever stricter Euro emission standards.
Unfortunately while the car industry rose to the challenge of making diesels less noisy and more refined, making a diesel less dirty has proved elusive. Diesels passed the weak and ineffective Euro tests with flying colours but in the real world there was no such thing as a clean diesel. There probably never will be.