I think you can drive a stranger's car but not a family car like your wifes? Open to correction though.
IIRC, I don't think we could transfer our NCB from private cars onto our taxis when we started so maybe the reverse is true also? One is a car vehicle and the other is a PSVehicle? Two different things in the eyes of the insurance company? I dunno though.
I think its another way for the insurance companies to rip people off, like if im driving a private car in Ireland for 20 years, no claims in that 20 years, paying say 400 euro insurance, move to Australia for 3 years, when I move back to Ireland, my no claims bonus is gone and I will have to start at the beginning and pay 3 or 4 grand for insurance, doesnt make any sense, its just greed from the insurance companies.
it makes sense that taxi insurance is higher than private because you have customers in the car who might get hurt and you are on the road way more and so your no claims bonus on private doesnt count for anything but a private car should be less of a risk? so your no claims bonus on taxi should count in a fair world.