I suspect this would only apply to roads that the Borough is responsible for, so not the Red Routes, including the A2, A20, A21, A205 (South Circular) and the A205 which are the responsibility of TfL.

According to government statistics, average speeds in inner London are 11.2-13.7 MPH, so would a blanket 20 MPH limit make any difference? I'm not sure whether it would result in traffic calming or increased road rage and those idiots who race round narrow roads at 50 MPH will do so regardless of the speed limit.