I was asked to be an external examiner in a math PhD viva at a UK university. I have never been to a UK defense and cannot find specific regulations/dress code guidelines on the University website.
What is the norm for what you wear in such events as an examiner? Is it usually formal or casual? My overwhelming preference is to wear casual unless that would offend someone. Would that offend someone?
UK vivas are not generally ceremonial in nature (the ceremony is left for graduation). As such, most people will dress more or less "normally". Especially in mathematics, I would expect most examiners to be dressed in a relatively casual manner.
In essence, treat the viva as an important meeting with fellow academics. I wouldn't recommend wearing shorts and flip-flops but, unless the maths department you are visiting is unusual in this regard, business-formal clothing would probably not be expected.
For almost all UK universities, Louwe's answer is correct. The only exception I am aware of is Oxford¹, where the candidate and internal examiner are expected to wear academic dress (smart clothes and gown). This does not apply to the external examiner, who will often dress as they would for a viva elsewhere in the UK, but if you happen to be examining at Oxford you may wish to be aware of this idiosyncrasy and dress a little more smartly.
¹Cambridge is not like Oxford in this regard
My view is that you don't want to make the candidate uncomfortable because they are either dressed too smartly or too casually. So I tend to be smart-ish, e.g. a suit but perhaps not wearing a tie if they are scruffy. I prefer not to be smartly dressed if I can help it, but it seems disrespectful to the candidate to turn up scruffy if they are smartly dressed, so I make a bit of an effort. Note this is a "special" day for the candidate, even if it isn't for you.
(but otherwise I agree with @Louwe, +1)
