I don’t think that emissions come into the decision to create 20mph zones. It is all about pedestrian safety:
Hit at 30mph 50% chance of living
Hit at 20mph 90% chance of living
Some details here:
The Mayor and Transport for London (TfL) is committed to Vision Zero, eliminating death and serious injury from London’s roads. The Vision Zero Action Plan sets out our ambition and next steps to achieve this. It details our plans to reduce road danger, including proposals to implement safer...
haveyoursay.tfl.gov.uk
The N. Circular and S. Circular are completely different beasts, and other than making a near orbital road, bear little in common.
All (most) of the NCR is purpose built, sometimes grade separated, with properly designed junctions. Little of the SCR is purpose built, it is mostly a hotch potch of pre existing roads and includes delights such as the inaptly named Rushy Green gyratory (you cannot rush it, and the “green” is now an electic mix of businesses including McDonalds, Lidl, a rather good pub, and a now closed down gun shop below a tower block called Eros House).