There must be a department policy on restructuring and it must be consistent
The only valid reason that I can think of for department A to be considered and not department B would be if the change included relocation beyond a specific distance and the affected grades were non mobile grades
(That would be that the new structure would be at the location of A or a new location C that is within relocation distance of A but too far for the relocation distance of B)
Unless that is the case I would think that there is the potential of a grievance against the process that has mapped one team but not let the other team be eligible
A grievance does not require union involvement (but it helps)
Help via the union is as good as the union volunteers locally / regionally and with the potential of “more experienced” national reps.
ACAS as mentioned is a dedicated body with the expertise to advise and step in for arbitration if it gets that far
In my experience of the public sector (MoD) mobile grades would be management and non mobile grades would be admin.
So all management grades in A and B would be equally assessed against the new roles, admin grades within distance would be equally assessed, admin grades out of distance would not
To assess the post mapping of a role you would take the new C job description, and compare that against old job descriptions of A and B. If they are identical or similar enough then all matched existing A & B staff map to the new C job. If too many map over then there are “limited competition” interviews for the individuals who mapped, if there are enough jobs for the matched then they map over
Potentially an old job could map to more than one new job
If individuals do not map (or don’t fit relocation distance rules) then they become surplus
“surplus” is technically redundant, but able to apply for other roles during the process.
As noted by the OP these may be “protected” and will typically have a surplus pool process to avoid the expense of actual redundancies (and avoid losing the headcount of people who could apply for other jobs in the department)
I have gone through quite a few MoD restructuring processes.
In one I was a “union observer” watching the post mapping comparisons. Groups of likely similar old to new job roles had already been gathered into folders in advance as part of the planning process, and I was “observing” the actual post mapping day to see that all were given a fair comparison.
My only recommendation was that I spotted two “different new job descriptions” which were only being mapped against the relevant old job descriptions, but they were both effectively two similar processes triggered by two different topic subjects on their own forms - I suggested that taking away the form name made the two job descriptions very similar, and the panel agreed which meant the groups could both map to two jobs in the new structure (it still meant less jobs in the restructure but those individuals mapped to 2 of them, still with competion interviews but 2 chances of success
In most of the others I was in the management teams running the restructuring and mapping, and in different ones I was doing the initial structure and picking the groups of old / new roles to be compared, or participating in the post mapping panels