surely any strike action would result in BA having to compensate NCL for failing to provide a service under their partnership
Afraid not - not unless it's a commercial agreement between them. Tour operators are absolutely shafted by the law in these circumstances, they have to pay compensation for all sorts of things that are beyond their control, and then it's down to the goodwill of the carrier to refund them. The most they'd get would be the cost of the ticket back - all additional costs would be borne by them alone.