A new tiara was bought for Sarah, and Sophie was given one made out of leftover bits found at the bottom of a safe somewhere (at least that how that ugly thing looks to me).

I believe Diana was given her tiara because it was expected it would eventually revert to the collection (ie: in time, D would become Queen and the tiara would stay in the collection). All the other wedding tiaras were loans. Even Anne's wedding tiara was a loan.