Apologies in advance for the slightly sarky response, but I’m still not convinced why the laws of physics need to wait for being confirmed by a DfT study.
The correlation between injury rates and speed being hit with is well studied and accepted around the globe. That being said, I’m in no doubt that a study could be crafted to prove exactly the opposite should this be the intention of the one asking the question, as is the case with everything.
I trust that there is a chance indeed that accidents actually rise if there is zero enforcement of the 20mph, as it may lead to increased levels of reckless driving. Reverting back to 30mph because of this would be a defeatist conclusion in my view.
There is of course the question about what is proportionate. I would argue that this should be answered from the viewpoint of the one being exposed to the risk rather than the one posing the risk. Where there are many pedestrians and not enough protected crossing facilities, 20mph is most likely to make a material difference to safety. Unfortunately, this situation applies to most roads in the borough regardless of how important they are to motored traffic. Even the South Circular is technically a residential road, as it has as many residents living and shopping either side as other roads, if not more. One could argue that an alternative to 20mph would be to make people to use pelican crossings only, “enforced” by fencing off all other areas where one might wish to cross. This is pretty much the solution adopted decades ago, although many London areas still suffer from the blight this has caused. The arrangements on the South Circular as a half-way house work, but not very well, and should really form the exception rather than the desired standard.
A further solution would have been to separate out trunk routes entirely from where people move on their feet, but we’re around seventy years late for this so the ship has sailed. Any attempt to rectify this now (anyone remembering Boris’s road tunnels under Central London?) is doomed for glorious failure.