Here's an easily understood diagram from MoD literature.
Class A risks are Unacceptable. Class B and C are Tolerable, if ALARP. Class D are Broadly Acceptable.
In other words, any risk where the severity of harm is "catastrophic" (i.e. loss of aircraft) must be reduced to ALARP even if the probability of occurrence is "incredible". But, as JFZ says, individuals projects can apply their own criteria, within the boundaries of the regulations. Specifically, this does not include "It's never happened before, so won't happen".
When you map the various risks noted in BAeS and QQ reports, you'll find there are a number of Class As that were not mitigated to ALARP.
