TCLS considers that let bygones be bygones. The individuals who obtained artefacts from the colonial times are no longer here to decide whether they should trade these with anyone. No one has the right to dispose of the artefacts to suit diplomatic imperatives today. Each country is on its own. The people of Greece today do not have an inheritance right to the Elgin Marbles because they were sculpted a very long time ago by artisans who are no longer here to demand their return. Governments come and go and need to stay off the idea of developing the cultures of the new generation which should left to the populations today to enact.
Elgin Marbles are only a bit of stone, not worth fighting over except that finders keepers principle applies. The United Kingdom found them, they belong in the United Kingdom's museum.