It's not nearly as blanket as is being implied, and clearly not the issue at hand, but

From https://www.gov.wales/introducing-default-20mph-speed-limits

Wales was one of the first countries in the world, to pass a law changing the speed limit on restricted roads to 20mph.

Restricted roads are usually in residential and built-up areas where there are lots of people. They often have street lights on them, placed no more than 200 yards apart. Historically, these had a 30mph speed limit.

48km/h reduced to 32km/h.