From the CATS OPS PART 121. "In these circumstances the maximum normally permitted is calculated according to what actually happens, not on what was planned to happen."
This interpretation seems to be very vague, but generally concensus is that knowing that one is going to leave base and go into Capt's discretion to get back is illegal. I wish this could be spelt out more clearly by the CAA. I think the intention of captain's discretion is to get back to base when stuck out an outstation after some type of delay, be it technical or wx related.
If I had a co-pilot that wanted to get off, I would ask him to give me his reasons, and I might remind him that it is the captains discretion alone to exercise discretion. The captain has to consult with his crew, but no where does it says he has to reach a consensus. If he still wanted to get off, and I felt it wasn't appropriate, then he would have to explain on the captains report to the fleet captain why he wanted to get off.