Earlier we had been discussing which tiara the Queen might wear for the portrait. Well, I think it is so right for this portrait to be without any head ornament.
I am always astonished at the way some brilliant painters capture the full lustre of diamonds! The Queen is wearing Queen Victoria's necklace and earrings, which are now known as the Coronation Necklace and Earrings. The robe is the State Robe, worn on the way to a coronation, and worn at State Openings of Parliament.
The full title for the portrait is: The Coronation Theatre, Westminster Abbey: A Portrait of Her Majesty Queen Elizabeth II.