Why the destination decides
South Sudan has not joined the Hague Apostille Convention, so a British mark alone does not answer for a paper used there. Records going that way need embassy or consular legalisation instead: certification by the FCDO Legalisation Office, then a consular stamp after it. That pairing is fixed by where the paper is going, and by nothing else about it.
Where South Sudan stands
An apostille is not accepted for South Sudan. It is a statement about a country rather than about a document, which is why it holds equally for a marriage certificate, a transcript and a board resolution. People often expect a rule of that kind to carry exceptions for particular sorts of paper. This one carries none, and planning around an imagined exception wastes the whole run.
The one part that does vary
The starting sheet is the variable. Where the record is a birth, marriage or death entry, Nottingham Register Office issues a certified copy, which keeps your own certificate out of the post altogether. A degree, a company filing or a court paper is obtained from whichever body issued it, and that step alone can take longer than the rest of the run put together.
Once the right sheet is in your hands the differences stop. Both stages ahead act on the document in front of them in the same order, at the same published price at the British end. Anyone who has taken this road once, for one sort of paper, already knows how it will go for the next one.
Cost and timing, paper by paper
The published rate does not vary by kind of paper: £45 / document standard postal, £40 / document next-day for registered businesses. What varies is how many documents you have and how fast each source can produce one. The consular charge and the pace behind it sit outside all of that, and are settled well away from Nottingham.