Final answer:
The phrase indicating the success of British propaganda in causing public sensation is the mention of the execution's impact on the press and public opinion.
Step-by-step explanation:
The line from the text that suggests British propaganda aimed at angering the public had been successful is "The shooting of an Englishwoman ... for treason has caused a sensation." This indicates the extensive reaction and emotional response of the British and American public to the execution of Miss Cavell, possibly fueled by the propagandistic portrayal of her as a martyr, which was meant to galvanize public opinion against Germany during World War I.