As of November 2015 Lambeth will be rolling out a borough-wide 20mph speed limit on all borough roads.  The implementation should be completed by March 2020. The new speed limit will be marked with signs and road markings rather than physical interventions.  

Lambeth Council will however be monitoring the speed of traffic and if after 18 months the speed of traffic is unacceptably high in certain roads, the Council will consider introducing physical measures to reduce vehicle speed.

Please see the below links for further information:

http://love.lambeth.gov.uk/lambeth-roads-20mph/

http://www.lambeth.gov.uk/parking-transport-and-streets/streets-and-roads/lambeth-goes-20mph-guide