Easy answer to the OP is that "It's the law, innit?"
The Civil Aviation (Investigation of Air Accidents and Incidents) Regulations 1996 (Civil ac)
the Chief Inspector by the quickest means of communication available and, in the case of an accident occurring in or over the United Kingdom, shall also notify forthwith a police officer for the area where the accident occurred of the accident and of the place where it occurred.
or
The Civil Aviation (Investigation of Military Air Accidents at Civil Aerodromes) Regulations 2005 (Mil ac)
So, since at least 1996, there has been a requirement in UK law to inform the Police at the earliest opportunity.