Oh dear! We have indeed seen this Oozlum bird circling a few times on PPRune.
The UK CAA consider 'Discretion;' to be a 'normal ' part of a duty day and that crews should normally be fit to extend the FDP. (I have it in writing) and "I don't want to do it" is not acceptable. I am not fit to is.
I was once with an airline that 'ordered' a Captain to exercise his 'discretion' - fortunately he was wise enough to ask for this 'in writing'. Heads rolled.
The CAA (supposedly) monitor discretion reports from any airline and if the number rises too far (I have heard 30%) on a route the company is 'advised' to change the rotation.
It can and is used for any reason including 'general operational lateness' on a rotation that should have 'worked' ie is 'exceptional'. I have used it a few times, both in and out of base.