Not going to waste time repeating previous comments but...
..calling a major incident plan as a constable first on scene.
In the paramedic/ambulance staff role the first attendant on scene will assume the role of incident officer until relieved by more senior staff and certainly could/would 'suggest' the instigation of a major incident plan based on what they see.
Also bear in mind a major incident is an incident which 'affect the normal day to day provision of ambulance/police/fire/etc services' so varies considerably depending where in the UK you are...
PW