KelvinD,
If single engine approaches over motorways are to, standardly, close the motorway then LGW and the M23, LHR and the M25, SOU and the M27, STN and the M11, to name but four and as for Duxford and the M11, well does Duxford get much traffic with more than one engine?
Whether it be a PAN or a MAYDAY it's still a '7700' squawk that warns all others to stay away, it's been suggested that a MAYDAY can always be downgraded at a later time, yes and a PAN can always be upgraded at a later time.
It might be discretionary upon the aircraft commander whether he declares a PAN or a MAYDAY but, on the ground, not everything in life is discretionary, procedures are in place for specific scenarios, ... Emergency service to ATC "Did he declare a PAN or a MAYDAY?", ATC to emergency service "He declared a MAYDAY", emergency service to ATC "Holy ****"!