If a Cabinet or Exchange has previously been upgraded by BTO does that preclude the LA, in this case Wiltshire County Council, from ever touching it with BDUK or BDUK2 funds? I am vaguely aware there maybe some EU rules which would prohibit this.
It isn't surprising that you don't get a clear answer - because different councils apply different interpretation of the same rule.
It seems that Surrey County Council seem to apply the rule to mean that no property attached to a commercially-upgraded cabinet can have any money spent on it. This remains true, even if the property is too far from the cabinet to benefit from any VDSL2 service at all: Surrey believes the EU state aid rule means that the onus remains with the commercial rollout.
Meanwhile, Warwickshire take exactly the opposite view - that even a single property without access to a superfast service, even when already connected to an upgraded cabinet, is enough for the area to be considered NGA-white. When accumulating results into postcodes, then the postcode is considered "white" even if only a single property does not have access. And when accumulating results into maps, the area is coloured white if a single property does not have access.
To some councils, the limitation comes about because of the way they declared areas as NGA-white in the original OMR. If they followed postcode boundaries in the declaration, it seems that some feel compelled to honour that declaration.
Some counties are then altering this inflexible approach in the second round - perhaps by declaring the areas in the same way as Warwickshire. Other counties are marking some of the areas declared commercial as "at risk" and retain the ability to re-label such an area as NGA-white if the commercial rollout fails to supply.
It probably comes down to wording in the OMR's, and what the council officials (at the time) understood about the limitations of both FTTC and the commercial rollout.