There is evidence that 20mph zones do save lives and injuries even if people don't obey them. This is because they still drive slower than they would in a 30mph zone.
I would agree, however, that if the limit is set to 20mph then the road design needs to be changed to match that, making it uncomfortable for drivers to exceed the limit. Unfortunately the UK is quite institutionally poor at this kind of traffic calming design compared to some of its neighbours.
You misunderstand the statistic. The 90% compliance measures cars that were driving into a future LEZ area. There was no need for any of those vehicles to be compliant at the time of measurement. The measurements were performed to analyse the impact that the new LEZ would have when introduced.
Additionally, I think it's extremely misleading to say that you need to buy a new car every eight years. In this case, you might need to buy a used car that is already eight years old (or much older if petrol). They are not expensive compared to new cars. In the case of London there has also been a scrappage scheme to assist people, though this probably isn't available everywhere.
Where is there an LEZ that forces you to buy a 'new car'? The ones I'm aware of require compliance with either fairly or very old emissions standards.
For example, in London, the ULEZ requires that petrol cars be Euro 4 (every UK vehicle since 2006 complies automatically) and diesel cars to be Euro 6 (mandatory for new cars since mid-2015). So, worst case, the vehicle needs to be newer than eight years old. More than 90% of vehicles driving into the expanded ULEZ area were already compliant before it became active.
Dubai isn't in Saudi Arabia.