Our company's policy is that the cabin crew can initiate an evacuation in case of a catastrophic event
Let's be clear; the decision to evacuate, together with orders on how to do that (which side, exits etc) can be taken ONLY by the aircraft Captain.
The catastrophic situation where cabin staff can and must initiate evacuation is essentially one in which the most senior person left standing aboard the aircraft is the most senior of the surviving cabin crew, who becomes the Captain by default.
This situation is so catastrophic that it is probably close to the situation in which any survivors would evacuate as best they can without waiting for anyone to tell them to do so; typically this occurs when the fuselage breaks up.
When a B727 crashed on landing at Doha in 1979, with the shattered remains ending up in the fire station, a passenger who was seated just behind the point of fuselage break-up undid his seat belt when it had all come to rest, picked up his briefcase, stepped out of the wreckage, walked to the terminal, and got a cab to his hotel. He was British, of course, and well-used to preserving his sang-froid when travel in the Gulf region did not go according to plan, ie situation normal. He saw no need to bother the very busy people helping to rescue other passengers. He was one of 19 survivors of 64 occupants, and it took 24 hours of searching to locate him, and then only because he rang Gulf Air to re-confirm his onward flight to Abu Dhabi.