The Royal Mail used Middleton's painting of Slieve na Brock in the Mourne Mountains to commemorate the Ulster '71 exhibition in a series of postage stamps that also featured the work of Tom Carr and TP Flanagan. In 1972 Middleton toured extensively with his wife visiting Australia for two months and showing his works from the trip at the McClelland International Galleries on Belfast's Lisburn Road the following year. In 1973 he also visited Barcelona and later showed a series of surrealist works inspired by the two trips at the Tom Caldwell Gallery in Belfast.
