That's the way it works with NCB, it can only be applied to one policy at a time.

There's a company/underwriters call Zurich that do policies where they will match the NCB discount from another policy.

Your dad will need to supply proof of his present NCB (the company should supply this on request) and Zurich will offer a disount on their policy. ie. If your dad has full/5yrs NCB, he'd get a discount equil to, say, 60%. Technically this policy will have no NCB of its own thou, but at the end of the year he's earn't 1yrs NCB which he can then take else where.

Don't know how competitively priced Zurich are, but could be worth a call.