I think that when you take out a fresh policy your NCB is verified by your last policy - you send them proof that you had X years and they accept that so none is lost.
Most companies allow 2 years, it's the reason that, now we are a 1 car couple, we reregister the car every 2 years, it seems very unfair, we were on each others insurance for 40 years and have never made an at-fault claim!
My father is 87 and car is registered in his name. My mother had always been named driver but has noinsurance or NCD in her own right, Insurance Company say when my father stops driving, mother will be able to obtain insurance in her name with full NCD.