This composer created the song cycle
The first three parts of this composer's Songs for Achilles were taken from an opera in which the titular Trojan is killed by Neoptolemus, while another of his operas sees Jo Ann fall in love with Merlin, who arrives in Act I on a spaceship.  This composer created the song cycle The Heart's Assurance for the tenor Peter Pears and may have used The Magic Flute as a basis for his first opera The Midsummer Marriage.  His oratorios include The Mask of Time and one featuring African-American spirituals such as "Go Down Moses" and "Deep River." That work's libretto was originally planned to be written in part by T.S. Eliot, but instead was written completely by the composer, who used Herschel Grynszpan's sparking of the Kristallnacht to express his objection to the Nazis. For 10 points, name this British composer of A Child of Our Time.
The first three parts of this composer's Songs for Achilles were taken from an opera in which the titular Trojan is killed by Neoptolemus, while another of his operas sees Jo Ann fall in love with Merlin, who arrives in Act I on a spaceship.  
Sir Michael Kemp Tippett