Where property is destroyed or damaged by fire, arson is committed under Section 1(3) of Criminal Damage Act 1971:

Damaging Property/Arson with Intent to Endanger Life/Recklessness

Section 1(2) of the Criminal Damage Act makes it an offence to destroy or damage property intending thereby to endanger the life of another, or being reckless as to whether the life of another would thereby be endangered.