If it's 30 them who makes the cut?
HMQ (1)
Children and spouses
Prince Charles and Duchess of Cornwall, Prince Andrew, Princess Anne and Cdr Lawrence, Prince Edward and Countess of Wessex (7)
Grandchildren and spouses
Duke and Duchess of Cambridge, Duke of Sussex (assuming Duchess will not travel at this stage of pregnancy), Princess Beatrice and Edo, Princess Eugenie and Jack, Peter Philips (separated), Zara and Mike Tindall, Viscount Severn and Lady Louise (12)
That leaves 10 places (assuming you don't have to count the Archbish and the Lord Chamberlain)
So who next?
Any other relatives of HMQ (she has 7 cousins still living)?
Other family of the Duke? (sisters all deceased, don't know if he was close to any nieces/nephews)