Soddim, if the MoD had merely approached the airlines to charter a number of aircraft to fulfill a task, the ANO requires the company to plan the task within the limitations of CAP371, and any further local agreements contained within their operations manuals.
To comply with these requirements would not require extra aircraft, and the extra cost of additional crew is minimal compared to the operating costs of the aircraft.
IMO, BALPA is being responsible here- by addressing the problem at tha planning stage, before the war starts, it can be sorted, and the (hopefully very few) casualties can be brought home with the level of safety they are entitled to.