Guests including David and Victoria Beckham, as well as George and Amal Clooney, have arrived.
The UN has passed a resolution to investigate war crimes committed by IS against the Yazidi people.
Hollywood star George Clooney says he is ready to become a father to twins, and has told the US site Extra: "I know what I'm in for."
The screen star, 55, and his English barrister wife Amal, 39, who have a house in Sonning, are expecting their first babies this June.
Mr Clooney told US site Extra that Amal was "doing great".
"She is amazing. I don't have anything to do. There is nothing I can do to help, but make tea and stuff," he said.
He joked that nappies will be central to his life, adding "for me, not for the kids".
The Hollywood star recently turned up unexpectedly at a care home in Berkshire, with flowers for 87-year-old fan Pat Adams.