Space Mercenaries by A. Bertram Chandler

Her ex-Imperial Highness of Outer Space had developed a conscience. With a well-armed space cruiser on her hands, she didn’t want to sell it to just anyone – that is, anyone under Empire control. So the former Empress and her ex-space captain husband became mercenaries for GLASS – the Galactic League for the Abolition of Suppression and Slavery.
Their first assignment was blockade-running, to bring antibiotics to the plague-ridden humans on Antrim, besieged by the Halicheki bird-people and ignored by the Empire. Only, this would be a ticklish business for they could not fire one shot at the Halicheki without being legally termed pirates. And although the ex-Empress and her husband were open-minded enough to try all sorts of devious tricks, the prospect of being hanged for piracy by the Empress’ successor did not appeal to either of their natures…