Fleur didn't wear the "bridal tiara", actually. If you look at larger pictures of the two you will see that they are very different. The bridal tiara is much bigger.
Fleur wore Queen Olga's small tiara. It was worn by Marie Therese at her wedding to the current Comte de Paris, as well as by Duchesses Mathilde and Marie (nee Wied) at their pre-wedding balls.
Sean