AUK cannot guarantee a brevet will be carried out following the rules and guidelines of a national lockdown, so staying local for instance, avoiding retail outlets in other areas etc.
If they can't guarantee that, then they can't cover the activity and risk being in a liable position...
I'm broadly sympathetic to this argument, but I would counter it by pointing out that there's already a large amount of trust involved in running audax events in normal circumstances - AUK is unable to guarantee that riders will follow the normal rules about riding on public roads during events, eg things like jumping traffic lights, using lights after dark etc etc etc. [ETA: reading back through the thread, I see Davef has already made exactly the same point!]
You can enforce the rules up to a point, but beyond that you just have to trust riders to be able to make their own decisions and behave as law-abiding citizens.
On the question of liability, is there any precedent for AUK being held liable for misdemeanours committed by riders on any of its events?