Cancelled Policy For No Valid Reason

My dad and brother recently insured a Mercedes CLK with them. My dad used his NCB and my brother a normal named driver. After around a months time they asked for No Claims Bonus proof which we provided. Everything was good until we received a letter to call them and discuss the policy. We phoned them and they said they have not received NCB proof which we said we will send again via email/post as we kept a photocopy and had already posted the original to them. They refused this and started asking my dad for Registration Numberplates for the other vehicles he has and the same for my brothers car. My dad asked why they need it which they had no answer for. He refused to provide the details stating theyre not insured with you so you have no reason to ask for them unless you give me a reason to provide them which again they refused to answer again. The phone call ended and within the next 3 days we got a letter saying the policy has been cancelled due to us not providing the details required. Are they allowed to ask for other vehicle details if not insured with them? Not a good experience with Churchill, first and last time we will use them.