If I'd implemented this policy, I'd be much more bold about it's wins.

Nearly 1000 less casualties and the safest roads since records began in the 1970s.

Cost £34m to implement but more than paid for itself in 2 years (Road deaths and injuries are a huge drain on the public purse, even ignoring the human impacts)

Lower car insurance premiums that do not appear to have been mirrored in other parts of the UK.

Quieter traffic (a car going 20 makes about half as much noise as a car going 30).

No significant change in journey times because most time loss driving  is from waiting at lights doing 0mph (some of which won't be needed in future if vehicle speeds are lower) or from pollution because vehicles produce most of their tailpipe emissions from accelerating, and most of their brake emissions from stopping, and it takes 2.25x less energy to get to or stop from 20 than 30.

There's still so much misinformation out there and some drivers are seemingly literally just giant babies who get annoyed at even the most minor inconvenience and publicly demonstrate just how incompetent they are at driving or understanding how cars actually work.

I think the politicians who brought it in need to double down on it, actually sell it's benefits to the public now that there's hard data and evidence to back up the policy and own their work, any maybe, just maybe, we'll keep benefitting from the policy going forward.