Please read these links regarding the use of Mayday and Pan, one is from the FAA the other from the CAA (UK.) Your view of the limitations of each phrase don't seem to be represented here.
From the CAA document:
"
An urgency message (situation
not requiring immediate assistance) is prefixed: PAN-PAN, PAN-PAN, PAN-PAN."
That is exactly what I said. A Pan call is nothing more than an urgent message. You can not expect immediate assistance, since you do not require it (that some ATC may give you the same assistance as if you declared an emergency, is because they are nice, cautious, bored, whatever... But you are not entitled to any priority or special treatment.
Why the reluctance to use a proper mayday like the crew of the US Airways did?
They wanted to descent and required immediate assistance in doing so. Why do you people have a problem with a mayday call. You don't have to pay for it.
Also please note there is no reference to any form of 'declaration.'
From your document:
"Fuel Emergency or fuel priority are not recognised terms. Flight crews short of fuel must
declare a PAN(????) or MAYDAY to be sure of being given the appropriate priority."
...thought I wonder how the English CAA would want you to declare an "urgent message".
Mayday = distress = emergency = something your can declare
Pan = urgent message = something you can't declare