Air Ambulance flights including Organ transfer are afforded priority A (highest priority) by UK ATC provided that "safety of life is involved".
This basically means that a flight rushing a heart to an urgent life saving operation will get highest prioprity but the flight bringing some young buck back from the ski slopes with a leg in plaster will not be deserving of the same priority.
Unfortunately, in both cases, the flight plan often will be annotated sts/hosp flight.
However the important thing is that they are totally different from an unexpected medical problem in flight.
I also can't see why crews declare a pan medical emergency and then bypass suitable enroute alternates.........I would not however say bypassing Gatwick to get to Heathrow makes much difference.
It would not be appropriate to declare a Mayday for an inflight medical emergency unless it is the pilots who have the problem because having someone sick down the back does not place the aircraft in "Grave and imminent danger" no matter how bad they are.
Regards,
DFC