Ok - mea culpa - I shouldn't have said "on the plus side" about the police being used to enforce this. What I meant was, in a situation where most drivers had been operating under the assumption that there was no sanction for exceeding the speed limit (ACPO and the Met had both said for a long time that enforcement of 20mph limits was an issue for local authorities and not the police) it was a bit pointless to have the limit.
As I've said before, I'd rather have a 30mph limit enforced and obeyed than a 20 limit which just leads to inconsistent speeds and dangerous unexpected behaviour as idiots try to overtake in unwise places.
For what I meant on police enforcement (prior to 2013, mainly), see http://www.telegraph.co.uk/motoring/news/10323354/Police-ready-to-get-tough-on-20mph-limits.html.
Unfortunately @chrisbeach the 20 limit will now mean (from http://www.cps.gov.uk/legal/p_to_r/road_traffic_offences_guidance_on_fixed_penalty_notices/)
The Association of Chief Police Officers (ACPO) has issued speed enforcement policy guidance, which suggests that enforcement will normally occur when a driver exceeds the speed limit by a particular margin. This is normally 10 per cent over the speed limit plus 2 mph. It also sets guidelines for when it would not be appropriate to issue a fixed penalty notice but to issue a summons instead (see below). Note that these are guidelines and that a police officer has discretion to act outside of them providing he acts fairly, consistently and proportionately.
Speed limit: 20 mph
ACPO charging threshold: 24 mph
Summons: 35 mph