It seems 20 limits cut accidents (who would have thought it?!), and so reduce insurance premiums: -
https://www.theguardian.com/money/2025/jan/18/uk-20mph-speed-limits-car-insurance-costs-premiums#msdynttrid=wFmgfJ_ax3HvjVbr7sVrg2498_esyCgx06WX8E88RM0
Cheers,
James