In June 2016 approximately 28 per cent of London’s roads had a 20mph speed limit which equates to 4,784 km of carriageway*.
Currently, as of March 2020, approximately 42 per cent of London’s roads operate with a 20mph speed limit, which equates to 7,430 km of carriageway*. This includes the successful delivery of 20mph speed limits across Transport for London’s road network in Central London, in line with my Vision Zero Action Plan commitments
I do not expect this number to change before May 2020.
Detailed queries relating to the speed limit on borough roads are best directed to individual London councils, who will hold more detailed information about their own road networks.
*the lengths quoted refer to carriageway length in each direction