In addition to his RAM duties, Armstrong was prominent in other aspects of British musical life. He was senior adviser to the Delius Trust, chairman of the Royal Philharmonic Society, a member of the Countess of Munster Trust, a member of the board of directors of the Royal Opera House, a Governor of the Old Vic-Sadler's Wells Foundation, chairman of the governors of the Central Tutorial School for Young Musicians (founded in 1962), now renamed the Purcell School, and chairman of the Musicians Benevolent Fund.