It is a 'semi-royal' wedding !
The tiara is a family heirloom. As Celia rightly said, unfortunately it is a bit lost in the veil frame. The bride wears it like her mother did
The tiara came to the Mountbattens through Princess Louise, Duchess of Argyll, daughter of Queen Victoria and aunt to Prince Philip's grandmother. Princess Louise had a soft spot for the Battenbergs and at the beginning of World War One, Victoria Battenberg was in Russia visiting her sister. Victoria quickly returned to England leaving her jewels in Russia for safekeeping. We all know what happened there. So, without much left in the way of jewellery, Princess Louise gave Victoria one of Louise's own tiaras. I have never seen a photo of either Princess Louise or Victoria wearing the tiara.
BTW, does anyone know the identity of the elderly woman whom Nicholas Knatchbull is escorting?