Not quite 'celeb' celebs, but I would imagine quite a few high flyers - breeders / owners / stud managers - from the racing world, so the Aga Khan and suchlike.
Lots of global heads of state, high-ranking civil servants / Palace staff.
Assorted aristos.
I should imagine it will be very formal and grave. Definitely no fucking Elton whinnying at the ivories, thank god.