I think that 20mph would be a very sensible limit in residential roads. Oddly the maximum throughput of traffic through a single lane and keeping safe distances is achieved at just under 20mph. That means that there would be better flow, less congestion and probably better average speed if it were implemented across London.
However, the weasel words in yet another duff Lewisham "consultation" is that there will be effectively no enforcement and the additional humps would have to do it. The humps are a pain, bad for drivers, really uncomfortable for cyclists and aggravating for residents.
There are many possible ways of encouraging/enforcing a 20mph limit - cameras, speed limitation on cars, advertising and official speed detection units in random sites.
However Government appears to be utterly lily livered on the matter, so the speed limits will be ignored, but Lewisham will be able to tick the 20mph zone coverage box on some audit or other.

I am all in favour of 20mph limits. However, I will vote 'No' because this isn't a serious attempt and I do not want even more road humps.