The British Crown Jewels were part of the ceremonial regalia of English monarchy. They were kept in the Tower of London. (COMIC: Secret of the Tower)
King John of England lost the Crown Jewels in the Wash in 1216. (TV: The King's Demons)
In the 1970s, Mike Yates of the UNIT said that getting a hold of the suspicious waste from the Global Chemicals would be have been difficult like stealing the Crown Jewels. (TV: The Green Death)
The Master once attempted to steal the British Crown Jewels. He was apprehended by the Third Doctor, Jo Grant and Brigadier Lethbridge-Stewart, and brought to the Tower of London. In an aborted timeline, he pushed Jo into a time scoop on his behalf. (AUDIO: Masterful)
Missy, using her vortex manipulator, was able to break into the Tower to steal the jewels. The Twelfth Doctor entered the room, barely missing the chance to stop her from again jumping into the Time Vortex. (PROSE: Dr. Twelfth)
The British Crown Jewels were given away as part of Concorpia's bankruptcy settlement with the Universal Monetary Fund. (AUDIO: Antidote to Oblivion)