It's not like the UK government is still tasking diplomats with buying random artifacts for the glory of the British Empire. There was a time when acceptable practices were different. We're not going to punish people for "crimes" committed 200 years ago, are we? We might as well disband the Catholic Church outright and pillage the Vatican, built on "ill-gotten gains" throughout the centuries (well, isn't that an appealing prospect, now that I think of it...).
Once you go down that route, could all museums in the world please send back their Italian Renaissance paintings, we better throw them in a dark closet at the Uffizi so nobody can see them, it's not like Italy has so many of them that they can afford to exhibit and preserve less than 10% of them at any given time, right?
In most cases, these "ownership disputes" are just instruments of foreign policy. I'd argue that preservation and availability should trump "ownership" pretty much all the time.
Going back to the preservation aspect: think of what's happening in Iraq and Syria at the moment. Historical artifacts of immense value are being lost to contemporary squabbles. Would it have been so bad to store those in a basement of the British Museum, away from the madness of humanity?