If they put up a ton of 30 signs then it probably would be alright. Eg, the main road through a village or town could stay 30, while the main high street and side roads would all be default 20. But that requires more than just a change in law to say “what was 30 is now 20”, and they don’t seem to be doing this. They’re expecting financially strapped local councils to go through a process of assessing and assigning 30 limits themselves, at their own expense. It likely won’t happen in most places, they simply can’t afford it.