It’s worth mentioning that Greece at the time was ruled by a foreign power who were more than happy to sell Britain the marbles, they were just planning on grinding them up to make concrete anyway.

It’s little different to if IS or the Taliban had allowed Americans to take significant parts of Palmyra or the Buddhas of Bamiyan before they blew them up - the ruling authority didn’t represent the people, but were still planning on destroying their history. Better to have it safe elsewhere than destroyed.

That said, there’s little reason for the British museum to still be keeping them.