Earlier quoted context omitted.
I was under the impression that most museums around the world are a cost to governments rather than a boon, there's probably a few exceptions. People who are travel to see the crown jewels are going whether or not this one is there with it.
Especially when most museums and galleries in London (all of the UK?) are free to enter. It’s usually only touring/limited display special features that have a modest fee to see them.
Apparently, the Tower is not government property, but belongs to the Crown Estate.
(I suspect the British cannot return the Koh-i-Noor diamond because it's not government property, either.)