What is a PhD, and what is a viva?
A doctorate is awarded for an original contribution to knowledge, and the viva voce is the oral examination at which that contribution is defended — a process that differs substantially from every earlier stage of education.
What a PhD actually requires. Not coursework and not breadth, but a piece of original research producing findings that did not previously exist, written up as a thesis. The formal criterion in most UK institutions is a contribution to knowledge worthy of publication, together with evidence of independent critical judgement and understanding of the field.
How it is structured in the UK: typically three to four years full time; a supervisory team rather than taught modules; annual progress review; and — significantly — no taught component in many disciplines, which differs from the American model where coursework and qualifying examinations precede research.
What the viva involves. An oral examination, usually by an internal and an external examiner, the external being a subject specialist from another institution. It commonly lasts one to three hours and occasionally considerably longer. The candidate is questioned on the thesis: its rationale, methods, findings, limitations, and where it sits in the field.
It is a genuine examination, not a formality. Examiners read the thesis in advance and may probe any part of it.
The possible outcomes, which surprise people expecting pass or fail:
Pass with no corrections — rare.
Minor corrections — the most common outcome, typically typographical and clarificatory, with a short period to complete them.
Major corrections — substantive revision, with a longer period.
Revise and resubmit for a further examination.
Award of a lower degree, such as MPhil.
Fail — uncommon, largely because supervisors generally do not permit submission of work likely to fail.
In many continental European systems the defence is public and largely ceremonial, with the substantive judgement made beforehand — which is why accounts of the process vary so much internationally.
What examiners are actually assessing: whether you understand your own work deeply, can defend your choices, know its weaknesses, and can situate it in the literature. Knowing the limitations of your work is expected, and claiming it has none is a poor answer.