Think I've got to the bottom of it now, having referred to College of Policing documents: Road Policing (RP): Investigating Road Deaths
and CLEAR (Collision - Lead - Evaluate - Act - Re-open) Leaflet
To minimise delay and congestion, "the RP lead investigator should ensure that road closures are appropriate and proportionate to the needs of the investigation. Forces should
adhere to the CLEAR guidance in discharging this responsibility". However, that is secondary to the mandate to investigate: "The RP lead investigator must
begin a road death investigation by adopting the mindset of unlawful killing, until the contrary is proved substantially
. Collisions that involve serious, life changing injuries should be responded to, investigated and reported in the same way as road deaths (that is: by adopting the mindset of unlawful injury, until... etc.).
RP lead investigators should not release a scene until they are satisfied that it has been fully exploited for investigative opportunities. Advancing the investigation takes precedence over the need to reopen roads".
All entirely reasonable and necessary when there has indeed been a crime: recklessness, drink, drugs, distraction, unroadworthy vehicle, no license, etc. The police will collect the prime-facie evidence, plus enough further evidence to make the charge stick, and take notes of the obvious conditions and features of the road that might have contributed to the seriousness of it. It has to be done.
Problem comes when the accident involves a single vehicle and it was really just an accident. The 'crime scene' mindset leads the investigators into an exhaustive search for something, anything, they can photograph or bag up for court as evidence of wrongdoing. But since there's nothing to find, the road remains closed for somewhat longer than if there was
criminality to be prosecuted. A substantial waste of everyone's time.
It seems that many (most?) other countries start from the 'it's an accident' mindset, which changes if there's something obvious about the drivers, the vehicles (tyres, tachographs), or the apparent speed/paths to collision, sufficient to trigger an investigation. But mainly, they take photos for a narrative account of what happened, clear the road, and everyone gets moving again.
My rant of the OP was, if anything, about a philosophy of policing too much about catching and prosecuting someone, and not enough about supporting people and protecting society - as evidenced by their enthusiasm for a criminal in the hand at a road accident, and their disinterest in burglary and assaults where the bird, or the evidence, or both, have left the scene, rendering a successful prosecution unlikely.
I think I understand now. Rant over. Thanks to anyone who took time to read, and/or apologies for taking your time. I'll go back to polishing my Alfa now.