If Diana really wanted out of the relationship pre-engagement, all she had to do was talk to the media or someone else who would quote her, like her sister did. She had plenty of opportunites to do so as the reporters and cameramen were chasing her everywhere. She could also just have behaved a bit oddly or inappropriately in front of the cameras.
Why woudl she do anything like that? If she was unsure about Charles, the best thing to do would have been to quietly refuse another date...There was no need for inappropriate behaviour