I don't think anybody wishes to accuse any individual or organisation of any illegal acts without any evidence (or if they do then they are too sensible to actually write it). The police have described it as suspicious, meaning it could well have been arson. Whether they will find evidence of who committed the arson (should this be established to be the cause) is another matter.

Unfortunately this has been the fate of a number of conserved properties in London, while other fall apart. It makes me wonder what the point of conservation is, when the police do nothing about squatters and conserved properties fall apart due to neglect or 'accidents'.