Eugenie, the daughter of Prince Andrew and Sarah Ferguson, is the only royal family member with a public Instagram account.
The venue is where Harry, younger son of heir-to-the-throne Prince Charles, Wednesday USA actress Meghan Markle at a glittering ceremony in May and Eugenie is copying some of her cousin's plans for her big day.
Also among the the pageboys and bridesmaids will be the Duke and Duchess of Cambridge's eldest two children, Prince George and Princess Charlotte.
Following news that over 23,000 people had signed a petition demanding the British government not commit any public money to the Oct.12 nuptials, insiders say the princess had a "bridezilla meltdown" and the "wedding rehearsal from hell".
Harry and Meghan, a former star of the TV show "Suits", are also expected.
Eugenie has asked Princess Beatrice, to be her Maid of Honor, the palace said, while Brooksbank has asked his brother, Thomas Brooksbank, to be his Best Man.
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Before this major (and honestly unexpected) switch, Princess Eugenie had been wearing her hair in a brunette shade with warm highlights as of late, one that was strikingly similar to Kate Middleton's hair color.
The queen, who has recently returned from an extended summer vacation in Scotland, plans to host a champagne luncheon for the newlyweds shortly after the ceremony, and a second reception will be held that night.
And with the presence of so many royals - and a number of celebrity guests - extra security measures have been put in place.
The bad blood is believed to boil over at Princess Eugenie's October 12 wedding to Jack Brooksbank.
The groom denied that Eugenie has been a "bridezilla" during the wedding preparations.
"We are getting a lot of American and Chinese and Spanish visitors", Jag Khaira at the Nell Gwynn Tearoom told the AP.
But one thing we do know for sure: This new hair color will look even better topped with a veil during Friday's ceremony.