There aren’t any rules for the viva, not exactly. That’s not how they work.
There are regulations, specific to each institution, but broadly compatible across the UK.
And there are reasonable expectations from the general stories that are told and shared about viva experiences.
And there are local “house rules” that each department might have (around lengths, formats and so on).
There aren’t rules for the viva, not really, but there is lots of information available that can help people figure out how to play the game. Look for what you need, and pass on what you figure out after you complete your viva.