The National Atmospheric Emissions Inventory (published at http://naei.defra.gov.uk/data/data-selector ), funded by the Department for Environment, Food and Rural Affairs and the Department of Energy and Climate Change, estimates national emissions of greenhouse gases and air quality pollutants across all sources, including transport. These emissions estimates are updated annually.
Local authorities are responsible for reviewing and assessing air quality under the Local Air Quality Management system. As part of this assessment London Borough Councils undertake additional monitoring. Information can be found on their individual websites.
The Greater London Authority maintain the London Atmospheric Emissions Inventory. The latest version was produced in 2010; estimates of pollutants are included for the base year 2010 and projected forward to 2012, 2015, and 2020. This can be found here: http://data.london.gov.uk/dataset/london-atmospheric-emissions-inventory-2010.