The common factor among all UK PhDs I know of is that there is a rigorous viva at the end, where the outcome for the sudent is not a foregone conclusion. Despite the shorter overall duration of the PhD (~3 to 4 years typically), the oral examination can (rightly) cover material far beyond the scope of your thesis - if you are an expert in your field, you will be able to have a knowledgeable and informed discussion as a peer with your external examiner, who will be a recognised expert in the field. I'm definitely a big believer in the importance of being able to have a well-informed discussion around the area of your work, and actually found the whole viva process very enjoyable and cordial - a nice chat about the wider field, my and the examiners' own previous work, some debate of the merits of different approaches, and then onto a run-through of the thesis, chapter-by-chapter, skipping any pages where there were no points for discussion or contention.

Unlike European vivas though, there's no family or friends, no champagne corks being popped mid-defense, and no foregone conclusion of the outcome. I've been at European vivas with the family of the candidate preparing the celebratory buffet at the back of the auditorium while the questioning continues!