Julian Ku writes that Amal Alamuddin-Clooney visited Greece as part of a legal team working for the return of the Elgin Marbles to Greece from Britain. He doubts
"the strength of their legal case for the return of the Marbles. At the time the Marbles were removed from Greece, the Ottoman Empire had sovereignty over Greece and there is pretty decent historical evidence that Lord Elgin had their authorization to remove the Marbles, or if he did not have authorization, his removal was ratified by official acts of the Ottoman government...there are strong moral arguments for the return of the Marbles to Greece. But...I think they have a very tough case...
One academic article explains the case for England keeping the Marbles so:-
In the early nineteenth century, a British Lord removed much of the sculpture from the Parthenon...The Greeks wish to see the Marbles returned to the Acropolis...Rejecting the emotional appeal of the Greek position, Professor Merryman analyzes the controversy...on reasoned, principled grounds. He concludes that the Greeks do not have a legal claim to the Marbles...He rejects cultural nationalism as a basis for the disposition of the Marbles, because cultural nationalism expresses dubious values and is founded on sentiment...Under the general principle of repose, the Elgin Marbles should remain in the British Museum until the Greek government can offer more compelling reasons for their return.
Be that as it may, the Ottomans had something much more precious: the historic national homeland of the Jews. Not cultural nationalism was involved but a nationalism over 3000-yars old expressed in political, religious, cultural, archaeological and a variety of other experiences, customs and practices all recognized by the supreme institution of international law in 1922, the League of Nations. And not by just one "colonial power" who declared in 1917 that that should be a nobel aim but that League of Nations' decision had followed upon an earlier one - the decision of the allies who defeated the Ottoman Empire to affirm the right of the Jews to their homeland at San Remo in 1920, to wit:
The High Contracting Parties agree to entrust...the administration of Palestine, within such boundaries as may be determined by the Principal Allied Powers, to a Mandatory, to be selected by the said Powers. The Mandatory will be responsible for putting into effect the declaration originally made on November 8, 1917, by the British Government, and adopted by the other Allied Powers, in favour of the establishment in Palestine of a national home for the Jewish people, it being clearly understood that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine, or the rights and political status enjoyed by Jews in any other country.
I would think that those persons claiming a right of 'return' to a country that never existed, a land that had no specific boundaries and no form of administration, surely have even less a case that could be made than Greece does.
The Land of Israel is real property not just cultural property. And our towns, hills, valleys, rivers and fields are worth much more than those friezes and sculptures.