I think that Clementine Churchill borrowed all the tiaras she wore.
The photo you posted a link to is fascinating. I have been working on an item about the tiara worn by Churchill's daughter, Mary Soames, for the last couple of days. I have to flee the house now, but I will try to do a post on it tonight.
The occasion was the Nobel Prize evening in Stockholm, December 1953 when Churchill was given the prize for literature. He was unable to attend but Clementine Churchill accepted on his behalf and read his speech.
None of the reports I have read so far say what the jewels were --just a diamond tiara and necklace!!