Each dataset contains its own level libraries in the dataset dgnlib folder, for instance C:\ProgramData\Bentley\AECOsimBuildingDesigner V8i Ss4\WorkSpace\BuildingDatasets\Dataset_GB\dgnlib.
The librarie file names are visible in Level Manager and Level Display if you turn the File column on.
I would expect Arup and HS2 to prevent configuration changes at the user level. Modifying the dgnlibs is an admin role.