It is 'Captain's Discretion' as has been pointed out above but only after the Captain takes into account the fitness of his crew to extend into discretion.
To me, that means a subjective assessment of how fit the cabin crew would be to handle a pax evac if I stuff up the landing at the end of an extended duty ULR flight.
Can I respectfully suggest that you are putting a legal rod right up your own jacksy if you do not, at the very least, consult with the purser to get a feeling for the cabin crew's fitness to continue into discretion. If you force it on them, something goes wrong and, at the subsequent investigation it transpires that the cabin crew were not considered or consulted then you may well be digging yourself a very big legal hole.
In addition to the above, it is also good CRM and, imho, good command.