National Express operate only a token number of vehicles in order to have an operators' license. Obviously, there are group companies like Speedlink and Kings Ferry but they are not bound by such strike action.
The businesses who work for National Express are contractors NOT franchisees. They provide the vehicles and staff to an agreed spec and operate according to an agreed contract.
NatEx counter staff could strike but with the amount of online ticket sales and other agents, it wouldn't be a show stopper. Same goes with the staff at coach stations and arguably, they could be manned by managers for a day