It is not a matter of intent it is a matter of fact. If the building is demolished it is as if it never existed in Planning terms. How or why the building was removed is irrelevant. The enforcement action and prosecution is not for demolishing the old building it is for building the new one without permission. You cannot accidently build a new house.
On the point of insurance an entire building does not just collapse down to ground level. Even with a severe fire you still get a few standing walls. I suppose a severe gas explosion might feasibly take down a building but even then it just seems to blow the roof off and blow out all the windows and maybe 1 or 2 walls. It might well be more economical to demolish and rebuild after a collapse but you should get planning permission to rebuild before you demolish the remaining structure. Buildings insurance usually covers costs of professional fees for architects, planning permission etc.
There are definitions for how much standing structure must remain before it is treated as a new build for Planning, Building Regulations etc, I cannot recall the figures off the top of my head. The Building Act used to say something like a standing wall of at least 12 feet in height must remain for the structure to be classed as an existing building.
With this case I am struggling to imagine a builder so incompetent that they managed to undermine all the foundations of a building to cause simultaneous and catastrophic collapse of the entire building. How would that even work? You would have to have a team of guys digging under the foundations around the building simultaneously and coordinated so that they they all reached the critical depth at the same time? There have been cases where basement conversions have gone wrong due to undermining the existing foundations and whilst they tend to cause severe damage and catastrophic collapse I haven't seen one where they managed to bring down the entire building.
