Mars,
I think the AAIB, not knowing where the lower bug was set because of damage to it post accident (though thought to be 200' as the PIC called, but not acknowledged by the co-pilot with his set at 500') correctly stated that one could have cancelled the other as they had the same priority - "CHECK HEIGHT (pause) CHECK HEIGHT" takes a lot longer to say than "dubublub 100 feet". If the 100 feet had not sounded then there was something very wrong with the AVAD.
But I think you miss the point - the finger is pointing toward the lack of organizational material addressing how the crew should have reacted to such events. The description 'relaxed atmosphere' can also be construed in several ways. You say what you think should have happened according to your OM - AAIB says there was nothing clear in the procedures provided to this crew. Hence the call for re-writes of the OM. I don't think it is the AAIB that needs to brush up on its offshore knowledge!!