OSGB data does not store coordinate system information. A metadata file is required to properly georeference the OSGB files. This error message appears if no metadata file has been detected or the Anchor Point parameter has not been specified.
For the Create Integrated Mesh Scene Layer Package tool, place a metadata or projection file next to the folder that contains the OSGB files. Alternatively specify a metadata file or point feature class for the Anchor Point parameter.