when you and the plane are cat IIIa approved just go...
i have flown rightseat Bae146 with CATII and at marginal weather it was always captain decision.
the only thing below catI was that the PIC makes the takeoff and no reduced thrust at takeoff.
now i ride a turbo seneca and cheyenne III leftseat catI and i have to make the decision. at this metar and taf , especially in ambulance missions, we go. not pretty but true. and funny when the doctor in the back tells us he has a hard job with responsibility...