I've lived in Kent most of my life, and I've never heard a soul speak of its being a shire. Kelly or no Kelly. It's been Kent with suffixes such as ion and ium since before the Conquest - the Roman one, that is. Its regional capital took its name from the region, not the other way round. Durovernum Cantiacorum under the Romans, the fort by the alders belonging to the men of Kent. Cantwaraburg under the English, the fort of the men of Kent. It was not part of the re-organisation that set up the shires under the English kings.

Nor was Sussex. Or Northumberland, or Cumberland.