Adrian, the problem is policing, currently the police don’t monitor 20 mph zones and anyway the way the police patrol they effectively become 24 mph zones ….
I am trying to address this using an afforadable average speed system https://store.autospeedwatch.org/ this system costs around £2000 for 2 cameras and monitors average speed. It is noteworthy that on average speed fines the police do not apply the recorded speed 10%2 mph to the indicated speed limit, just the measurement error margin.
I am also campaigning for local authorities to raise the fines under the Traffic Management Act 2004: Statutory Guidance for Local Authorities outside of London on Civil Enforcement of Bus Lanes and Moving Traffic Contraventions act.
Your support for this campaign would be much appreciated I can be contacted via the Beech Parish Council Website.