When I bought this cottage the cast iron bath had a steel waste pipe that went to the outside through a damp (and therefore conductive) wall. About 6 inches from the pipes's hole and fixed to the wall was the hopper and metal down pipe that went into the ground. A path ( metal, damp stone / mortar, metal ) existed between the cast iron bath and ground. I didn't measure the impedance. To me that was a low impedance path making the bath a candidate for bonding.
( That bath has gone and the steel pipe that was holding up some of the floor boards has also gone. )
( That bath has gone and the steel pipe that was holding up some of the floor boards has also gone. )