Lets not go overboard here. If somebody was sent to a conference but then skipped it, then they should they should face the appropriate consequences, potentially including termination.

However, it's legitimate for questions to be asked by management as to whether the person really needs to go to the conference, or whether that's an excuse for a most-expenses-paid paid trip to a friend's wedding. Half of the Cabinet seems to have made some excuse to check out last year's eclipse for example...

It's also reasonable enough to not want to facilitate an employee rolling into the office on Monday morning with a wedding hangover, having barely slept on a long flight home. Some companies also will have liability / insurance concerns about their employees adding personal days to work trips, although that is more relevant to dodgier countries than England.

But if the person is genuinely needed at the conference, and is willing to take a few extra days holiday, then a good company would find a way to let it happen. Even if it doesn't "emotional and irrational" is only one side of the story.