Ooooh... bring back the hanging cages at Tyburn!
I've no problem with 20mph in many zones - the reason being, NO driver ever seems to adhere to 30mph ones, anyway. Just wait til you keep to 30, and you'll have someone up your jaxie in no time, trying to push you on. Many round here prefer to zip on up to 40 if they can get away with it. So, 20mph will see people driving at 30mph anyway.
I do think there's a blanket idiocy here, though: in Brighton, we have clubs that are open until 2.00 a.m., some even to 4.00 a.m. When it's the height of the tourist season, and we have an average influx of some 250,000 extra people in town, then it's quite reasonable to impose an urban 20mph driving limit 24/7. However, it's just not necessary when you live in a rural hamlet where the most excitement at night might be an escaped cow. True, it's sod's law that you're the one to find it, at 70mph in your Porsche Boxster, but overall, taking a blanket approach is unrealistic.