Every road out of the country starts the same way
An apostille issued in Brighton for use in Sudan is not accepted, so this file ends with consular legalisation rather than with the shorter certificate. That difference is real, but it lands at the end of the process. At the beginning, a paper bound for Sudan and a paper bound almost anywhere else need exactly the same thing: a British public record in a form that can carry an official mark.
Readers rarely see it that way, because guidance is usually organised by country and so presents the whole road as country-specific. In practice the first half is generic and the second half is not. Recognising which is which tells you what can be started immediately and what genuinely has to wait for decisions you have not made yet.
What the shared half consists of
It is mostly the sheet. For births, marriages and deaths recorded in this district, Brighton & Hove Register Office at Town Hall, Bartholomew Square, Brighton BN1 1JA issues certified copies, and such a copy is an official record in its own right rather than a duplicate of the one you keep. Nothing about it points at a particular country.
The shared half also includes decisions that feel like admin and are not. How many separate papers the file will contain, which version of a record is the relevant one, and who will receive the finished file all belong here. None of these depend on the destination, and all of them are cheaper to settle before a single envelope has been posted.
Where the roads actually split
The split comes at the last act. Some destinations take the short certificate and stop; this one does not, and a further consular stage completes the file instead. Document legalisation for Sudan therefore has one more body in it than the shortest routes, one more queue and one more journey in each direction for the envelope carrying your papers.
Because the split is late, the sunk work stays useful right up to that point. That is unusual, and it is the practical reason to think of the road in halves. An apostille from United Kingdom to Sudan is not what this ends with, but almost everything before the ending would have been needed whatever the answer had turned out to be.
Why the halves change your timing
The common pattern is to wait. Someone holds off ordering anything until the far end has confirmed a detail, then does the entire route under time pressure. Splitting the job removes that pause, because the first half never depended on the missing detail in the first place and could have been running quietly in the background all along.
It also makes a change of plan survivable. If the arrangements shift, the certified copies in your hands are still official sheets of British records, and the count and the contents of the file are still the count and the contents. Knowing what you need for an apostille for Sudan is not the useful knowledge here; knowing which half of the work is already banked is.