It's a legacy of Robert Ayling's wacky tails. There were many different designs based on art from around the world. They all carried appropriate titles such as "Whale Rider" or whatever. "Chatham" was the design applied to Concorde.
When it was suggested to Ayling (by Maggie T among others) that he'd screwed up rather badly, the Chatham design was progressively applied to the rest of the fleet as they underwent routine repaints. There are only one or two aircraft remaining in the old livery now and the "Chatham" aircraft no longer carry the title, except on the plaque inside the main door.