Soopra: too late for you I guess for but for others, most insurers will honour NCB that is 2 years old, so you could insure the car on a classic policy for 2 consecutive years, then on a normal policy for 1 year to refresh the NCB.
I'm in a similar position at the moment: the household has 2 cars and 2 drivers, both drivers having built up significant NCB. I've nearly done 2 years on a classic policy. At renewal time I'll either put it on a normal policy, write off my NCB and leave it on a classic, or do some jiggery pokery with the 2nd car alternativing between policyholders (fiddly, not if it's worth it).