Congratulations JR76 and Tiara Mania for identifying the Brocket family tiara.
I am not sure if the tiara originated with Angela, Lady Brocket (wife of the second baron) or with her mother-in-law, Dame Florence Nall Cain, who died before her husband was given his title of Baron Brocket. Florence died in late 1927; Angela married in 1928. I have not seen any images of Florence wearing a tiara. Ronald, the second baron, succeeded to the title in November 1934.
The earliest photo I have seen of Angela wearing the tiara is the one Tiara Mania posted. It is dated 27 April 1936.
The next photo I saw was taken at the State Opening of Parliament in November 1936.
As I suspected the tiara is a composite, and Angela, Lady Brocket wore the lower section as a bandeau on occasions.
This comparison photo allows us to see that the bandeau is the bottom section of the tiara.
The current bride isn't the first of the Nall Cain ladies to wear the tiara as a bridal jewel: it was worn by Angela's daughter at her wedding in 1958.