Eau de Boeing
The best guidance [UK] can be found in United Kingdom Aeronautical Information Circular P138/2012 "Emergency Descents in UK Controlled Airspace"
The P stands for pink...because it is safety related. The AIC will explain slight UK differences from ICAO and is worth a read; it also gives an explanation to some other questions.
I prefer the AIC guidance rather than the more sterile interpretation given in the UK AIP En Route section.... ENR 1.6 SSR Operating Procedures..... page ENR 1.6-3 para 2.2.1[a].
Sorry I can't do the cut and paste thing........but if you really want to know you will find it. rgds