They will never be enforced. Many police forces including our largest one, London's Metropolitan Police have formally announced that they will not attempt to enforce them. One reason is technical, the lower the speed being measured with hand held laser devices, the more inaccurate they become. And of course as mentioned, the police on routine patrols themselves won't observe them.
20 mph is too slow, i.c. cars in general are totally unsuited to maintaining such a slow constant speed, so speed creep sets in as sjpt posted. Oddly though, fully electric cars like my one are very suited to it due to their vastly superior controllability, so in a sense the 20 limits are a bit premature. In twenty and more years when the number of e-cars start to dominate, these slower limits will be more appropriate.
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