 In 2018, he appeared as series regular Gildas in "Britannia", Jez Butterworth's historical drama about the Roman conquest of Britain.Appeared as Samuel Washington in HBO’s Gentleman Jack, written by Sally Wainwright.Performing with the National Youth Theatre, Armstrong's roles included Gerry Evans in "Dancing at Lughnasa" in 1998 and Lieutenant Stedna in "They Shoot Horses, Don't They?"