The comic book used in the film is a 1970s reprint of an authentic U.S. comic of the period (probably the EC title Shock Suspenstories #12 with a cover date of Dec 1953/Jan 1954). The front cover and contents would have been virtually identical to the original, but the back cover of the U.S. original would have had a period advertisement rather than the single comics panel on the reprint, as shown in the movie.
The trivia item below may give away important plot points.
Derek Bentley was partially pardoned posthumously by Home Secretary Michael Howard, who said he should not have been hanged but was still guilty. Bentley's conviction was later overturned by the Court of Appeal, who said that he had not had a fair trial because of the behaviour of the trial judge, Lord Goddard. The film has been credited for keeping the case in the public eye.