When I worked in central London, Tower Hamlets, the road safety officers would come to talk to the students about paying attention - not using headphones whilst crossing the road, look for traffic not your mates etc ... anyway, they would always include “what car would you like to drive” and conclude with the fact that it was pointless getting a hyper car - Lamborghini, Maserati, Aston Martin etc as the average speed through central London was ......8 m.p.h.
So yes, the 20 limit is a gimmick unless you can use a bus lane where you might reach 15 mph