Of cause keeping it is worse. If the capital is too far away, why would London be better?

Exactly. We agree there then.

Countries are a social construct so instead of focusing on boarders, bring it directly to the cultural heirs.

I think this tricky. Usually, I think the Cultural heritage belongs to the countries from where the artifacts were taken, so that's where the artifacts should be returned to. Otherwise, How do you decide who to give an artifact to? Most inhabitants of central America share Mayan ancestry, and they no longer follow the maya religion.

I guess it's a case to case basis. I am sure the rare cases in which there is a dispute it should be left upto the countries or institutions that claim the artifact to arbitrate.