To those saying well ofc it is enforceable, just because there is a sign doesn't mean something is enforceable. I couldn't stick up my own speed limit signs and change the speed limit for example, and councils do get things wrong in the process for getting a speed limit changed. The wrong paperwork being filled out, signs not in the right places etc can all lead to a speed limit not actually being enforceable.
In the ACPO guidelines there is a good page or two explaining how the road should also be made to look like a 20mph zone and not just have a 3 changed to a 2 on the sign. The police are there to prosecute those obviously flouting the rules not to get everyone doing 30mph because no one realised it was a 20. Without something like speed bumps, narrow points on the road etc then everyone will just assume it is a 30, police don't want to be dealing with that.
Either way, if you see a 20 sign 99% of the time it is going to be legally enforceable and you should expect to get in trouble if you are doing 30, though you are unlikely to get caught due to the lack of policing these 20 zones. Hit a pedestrian at 30 in a 20 and you'll be in deep ****.