Doesn't say that and that is not the suggestion. The point and question is with respect to what Law says that the members of 3 out of the 4 unions involved and non-union members, have to wait for the 4th union to agree before getting their agreed pay rise?
There is no such law.
When a bargaining unit is created it is specified how that unit's pay award decisions are to be decided. A bargaining unit
may specify that each pay award needs to be agreed by all of the unit's unions. Where this applies the pay award is not paid to any staff until all of the unions have accepted it.
Alternatively, the bargaining unit may specify that pay awards are to be accepted or rejected depending of the decision of the majority of the unions (and this specification may include any weighting to be applied to reflect differences in union size). The largest such bargaining unit is probably that for NHS for staff on Agenda for Change contracts. This has 14 unions.
Where a bargaining unit is set up this way, each accepted pay award is paid to everyone, including members of any unions that rejected it.
Thus, if staff have to wait until the last union accepts the pay award then this is because of how that particular bargaining unit has been set up, not any specific law.