I detailed it more in one of my other comments and the government data and graphs can be found here, but yeah the real non-compliance happens when roads are reduced without traffic calming measures. Which basically shows that reducing the speed limit on its own does nothing but criminalise road users.
I doubt that noncompliance can effectively be used to deliver further measures beyond speed limit reductions. Rather, people are going to say “See, your blanket 20 limit doesn’t work, you should undo it”.
Ultimately I see this as a very cheap but ineffective method at achieving its purported goals, but it’s very visual and very cheap so politically it’s fantastic.