The State of Emergency described meets the definition of URGENCY not DISTRESS therefore the correct call is PAN PAN. Whilst this may be well understood in the UK some other States may not be quite as familiar with the meaning, though they should be. It would not normally be appropriate to squawk 7700 unless asked.
If the state of Emergency warrants a DISTRESS call (Life is in imminent danger) then you want to attract all possible attention. Make a MAYDAY call on the frequency in use regardless of who it is, they are likely to have your current details; Squawk 7700, Bells are wonderfiul things to get noticed. Only change to 121.5 if asked, or if you cannot raise anyone on the working frequency.
You can always downgrade an Emergency Message, if you don't declare one you don't get special attention!