But..the metal body of a kettle IS an exposed conductive part! It's a conductive part that can be touched that is not normally live, but can become live under fault conditions; because it uses current. A radiator however cannot become live under fault conditions because it is not an appliance because it doesn't use current. However, it is liable to to introduce a potential.
Nope. BS 7671 defines an extraneous -conductive-part as, "A conductive part liable to introduce a potential, generally Earth potential..." However, if the rest of the circuit has a different potential, then there will be a difference in potential.