There are regulations about specific materials and installation practices, but there is no rule that if builders follow those regulations they have discharged their responsibility. They have an independent responsibilty to make sure the building is safe.

In this case, as I understand it using the flammable cladding wasn't explicitly prohibited; it was effectively on a "use only if you have independently proved the way you're using it is safe" list.

So in this case I don't think there was a specific failure of regulations.

I think it was a failure on the part of the builders, along with something of a general understanding that "red tape" is something to be ignored.