The campaigning group 20's Plenty for Us has published a document describing the status of various 20mph schemes throughout Greater London and the policy decisions (mainly from the Mayor of London's office) that have been leading to their becoming increasingly widespread throughout the capital.
A factor affecting policy was the rise in pedestrian and bicycle rider fatalities and serious injuries which was noted in 2011, reversing a prolonged reduction. Reacting to this, the GLA Transport Committee has called for a Vision Zero approach to road casualties.
As well as the gradual adoption of borough-wide schemes (see the map), notable changes have included the spread of 20mph limits to parts of the strategic "red route" network and the introduction of 20mph limits on the revised road network between Tottenham High Road and Tottenham Hale.
Download the report: 20mph Speed Limits in London: Update December 2014