<Discussion point for you.........twin engined jet announces to ATC that an engine has been shut down, but no emergency will be declared and wishes to continue to original destination Heathrow>
And for further consideration in this context... the pilot has no choice in the matter of declaring an emergency. If he advises ATC of specific problems, ATC then decides what category of emergency will be declared. At Heathrow (and probably many other places), loss of 50% of power constitutes a Full Emergency or higher, irrespective of the aircraft type.
PAN or Mayday may decide the level of priority given to the flight by ATC but will not necessarily have a bearing on the alerting of emergency services - ATC will decide the category of emergency based on information provided by the pilot and call out the services accordingly. After that, the situation may only be changed by the Chief Fire Officer.