This short guide will explain the differences in Air Ambulance & Police callsigns that are used within the United Kingdom.
All Air Ambulance helicopters within the UK use the callsign Helimed. The ICAO 3 letter designator is written as HLE (UK HEMS).
Following this is a number that identifies the exact aircraft, for example Helimed 01 which is the Cornwall Air Ambulance.
The third piece of information dictates the nature of the flight that the aircraft is undertaking. Only one is used at a time. Let's take Swansea Airport based Wales Air Ambulance EC135 G-WASN as an example.
The two main types of flight are defined as Alpha & Echo.
Helimed 57 Alpha would be on an operational task, for example en route to an incident or transporting a patient to hospital;
Helimed 57 Echo would likely be the aircraft returning to base, having finished its task;
Helimed 57 would dictate that the aircraft is undertaking training, or on a maintenance flight.
Police callsigns differ from Air Ambulance callsigns in that there are two completely different types used. One is used for communicating with Air Traffic Control, and one is used by the Police Force to identify and communicate with the aircraft.
The callsign used to communicate with ATC is the word Police, followed by a number to identify the aircraft/force area(s).
The callsign used by the Police Force differs from area to area, although is usually one or two letters from the phonetic alphabet followed by a number. This number is related specifically to each force. However in the case of multi aircraft operations or consortiums where more than one county is a part of this can differ slightly.
The Metropolitan Police operate three aircraft, and are numbered Police 251/252/253 dependent on how many are in the air at any given time. The East Midlands Air Support Unit operate across Northamptonshire, Warwickshire & Leicestershire and use the PRN number of the lead force in the consortium - Leicestershire - and are therefore Police 22. These two examples are used for explanation only; there are others where these principles also apply.
As a standard example, the Western Counties Air Operations Unit will use the callsign Police 01 to communicate with Air Traffic Control;
But officers from Gloucestershire and Avon & Somserset Police Forces will refer to the aircraft as Quebec 99.
UK Emergency Aviation > Callsigns Explained