My guesses:
Camilla: Queen Alexandra's Kokoshnik, Vladimir Tiara or Queen Mary's Fringe Tiara
Kate (if attends, even battling cancer and debuting her CH insignia): Princess Margaret's Sapphire Bandeau or Strathmore Rose
Anne: her Festoon Tiara, Aquamarine Pineflower or her Meander Bandeau (yes, that worn by Zara on her wedding day!)
Sophie: her Aquamarine Necklace Tiara or her wedding Tiara
Birgitte (debuting her Garter sash): Gloucester Honeysuckle Tiara, Princess Alice Emerald Bandeau or Cartier Indian Tiara