The requirement for police attendance and action at an event is based on the need for the police service to discharge its core responsibilities:
preventing and detecting crime
preventing or stopping a breach of the peace
traffic regulation (only under statutory powers relating to events)
activating contingency plans when there is an immediate threat to life
Coordinating emergency response activities associated with a major incident taking
place at the event.
It is also widely accepted that the police have no general duty to preserve public safety (crowd
control) at any public event, except where there are imminent or likely threats to life. Legal
opinion suggests that the responsibility for public safety rests with the organisers of an event,
the owners of the land on which it takes place and the local authority if the event takes place
on a road.
http://library.college.police.uk/docs/npcc/NPCC-event-policing-2018.pdf