Not a fan of the British crown (why are they exempt from taxes during record high inflation) but who should they return the diamond to - Afghanistan (it was the Durranis that looted it from Nader Shah after he looted it from the Mughals), India (Maharaja Ranjit Singh took it as tribute from Shah Shuja after conquering Kashmir from the Durranis), or Pakistan (the Sikh Empire capital was Lahore and the Durrani Empire c…
AFAIK they need money a lot more than Britain, and tangible food and resources are a lot more useful than a pretty diamond.
Though I kind of doubt this will happen…