The Queen, who is patron of the prestigious Royal Academy of Dramatic Art (Rada), was accompanied by Kate Middleton at the reception.
Current Rada students joined actors including Dame Helen for a performance in the Palace Ballroom.
The actress performed a speech from Shakespeare's 'The Tempest', the BBC reported.
The guest list included Steve McQueen, Moore, Dame Angela Lansbury, Joan Collins, Michael Sheen, John Hurt, Alan Rickman, Tom Stoppard, Alan Bennett, Ralph Fiennes, Michael Gambon, and Lenny Henry among others.
Dame Helen was presented with an honorary fellowship award by the Duke of Cambridge at the Baftas on Sunday and Prince William joked about the actress playing his grandmother.
Speaking about her conversation with Kate, Dame Helen said: "She said 'my husband called you granny last night' and I said 'yes that was so sweet'.
