I'm in the UK and haven't been told whether I should prepare slides. Has anyone prepared slides for their viva? How many slides would be sensible? I'm a bit lost!

I don't think that's normal for the UK, at least not in my uni, so maybe ask your supervisor or assessors?

You should be told if it is required. I had to give a brief presentation at the start of my viva. I had 5 slides and very quickly went through project background, novelty, key objectives and key outcomes. It was really just a quick few minutes to set the scene before them starting to tear apart my thesis.

The defence of the thesis shouldn't require any external aids, as far as I know.