Since when has the UK become the arbiter and police of the World's Art. Is it also rescuing currently "at danger" heirlooms ? Who are they to decide that they g

Since when has the UK become the arbiter and police of the World's Art. Is it also rescuing currently "at danger" heirlooms ? Who are they to decide that they get to show it? For e.g. the Kohinoor diamond from India. Blatant theft. But sure done by a white colony so it must be fine. Saviors and all that.