This is a bit of a dark one but the idea struck me in the shower. What if it were Tony Blair who died in August 1997, instead of Princess Diana? Let's say the circumstances are basically the same: a car crash in the Alma tunnel, Blair rushed to hospital, dies at 3am. Nation wakes up to a dead prime minister at peak of his honeymood period.
Almost certainly, Gordon Brown would be the new PM, after a short leadership 'contest'. I don't see anyone else really having the balls to challenge Gordon at this stage. And Labour would use Blair's legacy to drive through the New Labour agenda without any real opposition, at least for a few years. But I'm not sure what the longer-term ramifications could be.