The 20mph is absolute nonsense, and will remain so, unless it is enforced, and enforcement is quite rare. I've just done a bus trip, to and from the local town, sitting at the front, of a double-decker, where I was easily able to see the driver's own speedo. Unlike car speedometers, these are accurately calibrated, rather than the actual +10%. Both there, and back, we progressed through several marked 20 limited villages. The 20 on both trips, was completely ignored, we were doing an indicated 30 and 30+ through most of those.
I live on a residential road, in a village, with a blanket 20mph throughout, and at the moment and for several months, they have regularly been closing the busy railway line down, running a rail replacement bus service, down our residential street. We have two tight bends in the road, which are a precise 100yard distance apart, with a T-junction another 100 yards beyond. From my home, I can easily see, and time their progress twice over two chances. From the timing, I can easily calculate their average speed over the distance.
I often time them as doing in excess of 30mph, these are large luxury, single deckers, sometimes double-deckers tearing along a residential road, often with parked vehicles, and drivers reversing out of their drives. I have complained to the authorities, but no one seems to care.
The 20 limit is not there for safety, or to improve the local amenity - it's there simply as a money earner for local authorities, to gain points, and top up their funding when they need to.
They might be professional drivers, but a bus can do an awful lot more damage than a car...