PSZ is always measured form each depatrure end of the runway. It is a permanent designation at eaither end of each runway. Smaller airports ( less volume of flights) have a triangle with base 230m either side of centreline coming to an apex at 2.3km.
The ravine is outwith what would be a CAA designated PSZ ( public safety zone) but within Airport perimeter - no access to Public. ThPSZ is designed to identify areas of risk ,for departing and, I assume, arriving Aircraft. Within the bouandary there are inherent risks around the airfield - these whoule be assessed and mitigation measures employed to reduce or remove the risk
My point was that if the MD 80 left LHR 09L and followed the same path it would end up in the BMi Hangar!