Croydon-wide 20mph road traffic speed limit

[Robertj21a]

If that is the case then most drivers are blatantly ignoring the limit.

They may well be exceeding the limit. That's not the same thing as having to check their speedo all the time. I know when I'm exceeding a, say, 30 limit by about 5mph in much the same way that many cars on motorways routinely travel at 80 when they know that the maximum is 70. Many/most people in practice tend to drive just over the speed limit, but within the well-known tolerances usually applied by the police.