It used to be the case (AFAIK) that a 20mph zone *had* to have traffic calming measures applied as well, speed bumps, traffic chicanes or at the very least the thick traffic calming paint stripes. This in a sense enforces the 20mph somewhat, particularly the speedbumps.
However, at some point the policy has changed, and traffic calming measures are no longer required, and as such, the entirety of the measure is to erect signs, which require the same level of enforcement as any other speed limit, ie, quite a lot if you want people to actually comply (hands up who has never broken a speed limit(not really rhetorical question)), as opposed to letting the speedbumps carry out compliance for you..
But of course, speedbumps are expensive, so........ let's just put up some signs.... and tell people we are making the roads safer. i think it's better than nothing, but not as good as a physical measure which is hard to ignore.
Of course, one counter-argument is that speedbumps also slow down ambulances and fire engines, and perhaps you'd like them to be able to break the speed limit occasionally.....