Live in Wales so I have skin in this game...
Observations: 20mph limit means most people drive below 30 in urban areas.
20mph might not be universal in UK but many many places I drive through over the border in Engerland have a 20 limit.
Most roads with a 20 limit have an average speed of... Stationary /walking pace anyway.
The mandatory 20 limit was proposed and supported by Welsh right of centre politicos who dropped it like a hot potato on implementation - it was was weaponised legislation used as a stick with which to beat the left of centre Welsh Government Assembly ruling party.
My commute of 16 miles each way, though mainly through urban areas, has about 500m of 20mph limit.... Commute time is unaffected. I'm all right Jack!
Over all I like the 20mph limit and the shouty loudmouths who don't have failed to engage critical thinking.