Air Pollution Peaked Around 2003 and is Starting to Reduce

DEFRA released a report in April 2014 called Air Quality Statistics In The UK, 1987 to 2013 measuring air pollution since 1987 to the present day.

It's a mixed bag of results, but in general air pollution in the form of particulates (PM10) and Ozone peaked around 2003 and is slowly being reduced. The main exception being urban ozone levels, which have continued to rise:

If we measure that in terms of the number of days per year when air pollution is 'moderate' or 'high' then we can see a steep decline since 2003 to levels below where we were in 1987, despite there being many more cars on the road than there was back then:

Particulate matter, nitrogen dioxide and sulphur dioxide are down hugely since 1992:

One of the reasons for this is that cars are vastly cleaner than they used to be. The amount of CO2 released by cars has reduced by 15.1% since the year 2000, despite the overall number of cars increasing:

Source: Department for Energy and Climate Change (DECC)

The following graph from the Department for Transport (DfT) shows how CO2 emissions from all cars in use have declined relative to the total number of miles being driven by Britain's cars. Driving style, vehicle maintenance and road conditions also strongly influence total emissions:

Source: Department for Energy and Climate Change (DECC)

Another view of this is from DfT figures released in December 2013 which show that the average CO2 emissions of a newly registered car has reduced from 177.8 g/km to 128.4 g/km:

Source: DfT Statistics