The 1917 gold sovereign struck in London is considered the rarest Sovereign of the 20^th century, due to a combination of factors including limited production, historical events, and melting for their metal content.

During World War I, the British government issued bonds to raise funds for the war effort, many of which were purchased by American investors, who were paid their interest back in gold. The gold that was shipped from Britain to the United States is likely to have contained gold sovereigns, but it is difficult to know for certain.

It is also possible that some 1917 London sovereigns were sent to the American government to pay for war debts, as gold sovereigns were commonly used in international trade at the time

These factors contribute to the scarcity of the 1917 gold sovereign struck in London and consequently make it a highly valuable and sought-after coin among collectors.