As I understand it, (I'm licensed to drive here, but I don't) the way this went down was:
1. Government adds more safety equipment to some test Motorways which allows monitoring lanes and separately varying limits per lane. Unsurprisingly this makes these motorways safer, let's say a factor of X.
2. Government converts hard shoulders into running lanes to save money on some test Motorways. Unsurprisingly this makes motorways more dangerous, let's say by a factor of Y.
3. Government argues that X/Y > 1, therefore adding safety equipment and converting hard shoulders into running lanes, would be on net an improvement. Conveniently the fact that they're doing this because it's cheaper is not mentioned.
We don't accept this sort of arithmetic in other contexts and we shouldn't here. Maybe adding safety equipment is too expensive to justify the reduced danger, but moving to all-lane running is unacceptably dangerous, so, you can't do that, regardless of whether somehow then the safety equipment is magically free.
Previous governments have chosen not to take up obvious safety improvements because they were too expensive. For example, trains in the UK all have the very simplest and oldest SPAD-prevention, the Automatic Warning System, which is activated by an electromagnet (well, technically a magnet which is deactivated by an electromagnet so as to fail safe) but doesn't distinguish between Danger (do not pass this signal) and Caution (expect to find the next signal at Danger) so drivers can get into a bad habit of reflexively cancelling it since they need to pass Caution signals on many routes all the time. Government could have required Automatic Train Protection, a more sophisticated but expensive system which was demonstrated on a small fraction of UK lines and they elected not to spend that money to do the rest. Today most passenger trains have TPWS (which is cheaper than ATP but fairly effective) and a few newer ones are starting to do ETCS (the European system which obviously has production scale benefits, you're buying off the shelf gear that's sold across the continent).
On the other hand, unlike not making them safer, making the roads more dangerous is clearly just a bad idea, and so it became politically harder to defend this practice, hence Rishi now scrapping it.