“Firstly I would carefully review the planned action and consider it in light of company SOPs, then aircraft operating limitations.
that's a good initial answer. However, if the falls outside these parameters, you can't just
leave it up to you to decide.”
. CRM (and common sense) dictates you both agree with a decision.
Suppose it wasn’t quite so severe, for example we were on an NDB approach where the minimums were 600ft, and the cloud base was at 400ft, and I said to you this is our home base, there’s no high terrain and I’m going to do it anyway. What would you do then?”
“I would voice my concern highlighting that this is plainly illegal, we are bending the rules and will be caught.”
What he's proposing is not actually illegal. As long as the visibility is above limits, you can make the approach. The cloud base is not a factor is making that decision.
However, I assume what he was getting at was what you would do if you were not visual at minima and he wanted to continue. The answer to this is clear and simple: "Go around." And this is as far as it is ever likely to get in your career, if indeed there was to be any hesitation about a GA, which would be unlikely.
But supposing there was, no matter what you can or can not see, if you hear "Go around," you do just that and discuss why later. Remember, you may see no need for it, but the other guy may easily have seen something you have not.
At this time, if he refused to GA, it's getting messy.
If still IMC, you have little choice but to announce "Captain I have control" and execute a GA (in my company, the use of the word "captain" denotes a very serious situation).
If you have become visual, are clear of terrain and are in a position to land, I would do so, rather than risk a fight over the controls. However, I wouldn't take off with him again.