I'm conscious that I don't want to risk giving an inaccurate comment that could have consequences so would suggest that for the official position it's best to check the official sources, here is one, scroll down to the section on 20mph
https://www.gov.wales/roads-driving
What I will say though is that this news of the lower speed limit of 20mph is not known to all and will come as a surprise to some visitors to Wales from Sunday 17 September onwards.
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we call it fishing not catching!