Why not? Almost everyone that had it before won it through conquest, but only Britain is being held to a standard where that is forbidden.
And also the other jewels stolen from South Africa. Or at the very least, don't keep them on the regalia.
Didn't they, you know, give up their empire? And invite their former colonies to colonize them, to the point that they are now a minority in their own capital?
If anything this should have taught them that, not only will it never be enough, but every act of atonement will count for nothing and be immediately forgotten and discredited.
If they give up the Koh-I-Noor to whomever plundered it before them, ten years later someone like you will be claiming that it is hilariously naive to believe they just gave it up.