See CAP413 (downloadable at
http://www.caa.co.uk/docs/33/CAP413.PDF ):
Radio Procedures – Practice Emergencies
Pilots may simulate emergency incidents (BUT NOT THE STATE OF DISTRESS) on 121.50 MHz to enable them to gain experience of the ATC service provided. Before calling, pilots should listen out on the emergency frequency to ensure that no actual or practice incident is already in progress. Practice calls need not disrupt a planned flight or involve additional expense in fuel or time since the pilot can request ‘diversion’ to his intended destination or cancel the exercise when necessary.
Simulated emergency calls must be prefixed ‘PRACTICE’ and should be brief, e.g:
‘PRACTICE PAN, PRACTICE PAN, PRACTICE PAN, LONDON CENTRE, G-ABCD’
The Emergency Controller will then indicate acceptance of the Practice Pan by transmitting:
‘G-ABCD, LONDON CENTRE, CONTINUE WITH PRACTICE PAN’
The Emergency Controller may instruct the pilot to call at another time, if the practice cannot be accommodated.
If a practice is accepted, the pilot should then pass his details. SSR Mode A Code 7700 should not be selected during a practice emergency exercise unless required by the Emergency Controller. Mode C should be switched on, if available.