According to
https://publicapps.caa.co.uk/docs/33...026Nov2020.pdf
States of Emergency 8.2 The states of emergency are classified as follows:
1. Distress A condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.
2. Urgency A condition concerning the safety of an aircraft or other vehicle, or of some person on board or within sight, but does not require immediate assistance.
8.3 The pilot should start the emergency call with the appropriate international RTF prefix as follows: 1. Distress ‘MAYDAY, MAYDAY, MAYDAY’
2. Urgency ‘PAN PAN, PAN PAN, PAN PAN’ UHF and VHF Emergency Service 8.4 The UK Distress and Diversion (D&D) Section is located at the London Control Centre. It is manned by RAF control staff who are assisted in the provision of an emergency service on the International Aeronautical Emergency Frequency 121.5 MHz and on 243.0 MHz by suitably equipped civil and military units and certain HM Coastguard stations. The service is available continuously to pilots flying within UK airspace who are in distress, in urgent need of assistance, or experiencing difficulties (i.e. temporarily unsure of position) which could lead to a state of emergency. The service may also be available for practices