In 1987 I began high school. There was one YA novel that the school librarian put on a special part of the shelf because it was "too frightening". If this was reverse psychology it worked because I borrowed it. I cannot recall the title. I didn't finish it. I got about 1/3rd of the way through/skimmed some of it/jumped to the ending. Anyway:
- It was set in England. It had a British author.
- The front cover showed a boy crawling out of a pit with a terrified expression.
- The plot was that two young British guys - teenagers - went around Britain tandem on one motorbike. They stopped at a beach and stumbled on a terrorist camp. One of the terrorists knocks them both out with rocks. They wake up in a cave tied to a bomb. They are told that a terrorist plot is going to happen in a week where bombs will be put on British boats and subs at UK ports to blow them up and hurt the British government. They are allowed to live until the plot is done.
- Then I skimmed/skipped [maybe the due date got close] and the final pages had one of the teenage captives escaping and climbing out of a pit like the front cover. He passes quasi supernatural beings real and/or imagined.
I have tried various book forums. No suggestion has been it. It was very well written. For instance when the guys get captured there is a two page description of the pain of being hit. It's a bit synesthesia like with pain being a colour. I wouldn't mind reading it again or seeing what else the author did. I feel that it was a male author.
This is a tough one. I don't really expect an answer; I've given as much detail as I honestly can.