Waist covering for 4x1 DB dinner jacket

[Melcombe]

Sort of regular shirt, sort of didn't care about the tie.
https://www.youtube.com/watch?v=q8kmdC1VBTI

That's interesting. The speaker there is Rory Stewart who has all the qualities you would hope for in a politician but which – these days – makes him hopelessly out of fashion.

His background would suggest that he knows exactly how to dress for the occasion and in this case the plain shirt and "carelessly" tied bow tie are carefully attuned to the context : while black-tie is worn at the Oxford Union, the idea is that you show that you follow the tradition with a slight reluctance - resulting in a studied scruffiness. Anyone who was to turn up in the exquisite black-tie you would reserve for a grand celebration dinner would be exposing their lack of understanding on that point. Such are the shoals and whirlpools of keeping up with the Etonian/Oxbridge set.

You might find some of his bio interesting: https://en.wikipedia.org/wiki/Rory_Stewart

And here he is suitably attired to pop down to the shops for a pint of milk.

[arch]

"the idea is that you show that you follow the tradition with a slight reluctance - resulting in a studied scruffiness"

I would add, for those who were not born on these shores, that this is the unconscious aim of most men in the UK. It starts with school uniform.