Where is the autopsy report on that?
Not sure you need one - they were clearly incapacitated or they would have shut down the engine. Unless they both had heart attacks or strokes at the same time, the most likely cause is head injury leading to unconsciousness. Explain the lack of engine shutdown another way if you prefer.
SAS - not sure you are getting the message that UK HSE is irrelevant in this case or any other similar crash.