Not pretending to be absolutely right, but here's my interpretation of the book:
There is no discretion with ULRs. Discretions are described in another chapter and are also clearly limited. There is a only "maximum permitted limit" for ULRs and that in itself is self explanatory!
After that there is only a waiver of legal limits. Meaning just that: You are outside of any legality, solely and absolutely accountable for it.
The GCAA handed responsibility for that waiver down to the FCP and by that absolve themselves of any liability.
The FCP will hand down this sole waiving responsibility to you as skipper and absolves himself and the company of any liability or responsibility.
He will sweet talk you with 'think of the poor passengers, think of the cost for your company and think of blah, blah, blah', but will leave you alone in the rain in case of the slightest mishap.
As will the poor passengers. They will find a sleazy lawyer to sue the last drop out of you and the press will have a field day with your irresponsible disregard of maximum permissible limits .....
Think hard.
As for rest, i can only think of one minimum, that's 10 hours in a room with a bed. That's what you will get on a ORD, so think hard again: Do you want this after 23 hours or after 15, then continue to destination and have the same minimum rest again and all within clearly defined legal limits?
A no brainer, at least to me.