At the depots I worked at the skilled (cat 4) staff were nearly all members of the craft unions, usually the engineering workers, the electricians, or boilermakers unions, the latter for the welders and smiths.
Your local reps would be elected from anyone, who then represented everyone, irrespective of union. At national level negotiations would be dealt with by “the confed” (the confederation of engineering unions) who used to, with the NUR, sit down with the BRB.
NUR depots I knew tended to be ones where unskilled staff were in the vast majority but even then you could find confed staff present. Likewise you could find NUR staff at a largely confed depot, especially if they been working in other railway departments. NUR representation amongst the non skilled Cat 1 to 3 staff was common.
Clerical, Technical and Management staff were mainly TSSA (I was a TSSA depot rep for nearly 10 years) unless it was an NUR depot. Supervisors usually stayed with their craft union. Poaching of staff by unions was prohibited by the inter union (TUC) Bridlington agreement and if it occurred and complaints were made to the TUC, staff had to be returned to their original union.