The British Empire didn’t personally steal the artifacts. Individual British people, one of which must have either liked the Catholic Church or maybe was friends with a priest or whatever, stole the artifacts. The British Empire just saw it as a happy accident that they got the real stuff instead of copies.

These are apparently from the acropolis damaged during the Ottoman-Venetian wars. That makes it very very likely it was smuggled out by the British or a catholic working with the British.