My main concern with this is that what you're doing is desensitising people from the speed limit.

I'm from a country that has arbitrarily defined speed limits and VERY low compliance rates compared to the UK (if you've ever been to Italy for example you know what I'm talking about). The nice thing here is that because the vast majority of roads have a speed limit that 'feels' appropriate (ie the road is designed for its speed limit), the amount of speeding I see here is negligible compared to what I was used to.

And generally here when the limit changes people comply to it because you can trust there's usually a good reason.

There's roads near me that are arbitrarily set to 30 (no pedestrian walkways, no side roads, but it passes near the back of houses and I assume they successfully petitioned the local authority to change it to 30), and traffic flow there is usually 40-45. I've never seen an accident there.

We have a poorly designed intersection not too far away and there's always accidents there to the point that there's now a consultation to fix it.

If this rule came to England, both these roads would be turned to 20, and that won't really be solving anything. In the first example I assume locals will still be driving 40, and it will create unnecessary overtaking because the road is wide and the visibility is good so it's not necessarily unsafe. But you've gone from a safe 40 road to risking head-on collisions pointlessly.