This extract from the earlier version of the website for the Royal Collection suggests the British Embassy -
From the e-gallery on 3 Oct 2007 R. & S. Garrard & Co. The Oriental Circlet 1853 Diamonds, rubies, silver, gold Made for Queen Victoria Made for Queen Victoria and originally set with opals, possibly to the designs of Prince Albert. It was altered in 1888 and again in 1902, when Queen Alexandra had the opals replaced by rubies. This tiara was a particular favourite of Queen Elizabeth, and she wore it for the banquet at the British Embassy, 20 July 1938.