Just a thought. The media is going on about oxy bottles letting go. But that far back in the cargo there are no oxy bottles, but there are cargo fire bottles.
No wanting to criticise but on several pics you can see aft of the hole one of the Green oxy cylinders still in place. It looks indeed likely a cylinder departed the aircraft bottom first, folding up that flap of skin and breaking free thru the stringers. There is even rumours of LHR doing work in this area prior to its DEP from LHR, unconfirmed....