Where Guinea stands
Guinea has not joined the Hague Apostille Convention, so a British mark alone does not answer for a paper used there. Papers going that way need embassy or consular legalisation, which means document legalisation for Guinea runs across two desks rather than one. There is no apostille issued in Southampton for use in Guinea that collapses those two into a single step, and no version of the file changes which desks are involved.
Two clocks, running in series
The order is not a formality. The desk that gives the final mark is reading the British certification already on the page, so it has nothing to work with until that stage has finished and the sheet has come back. Nothing can be lodged in advance and nothing overlaps. In planning terms an apostille from United Kingdom to Guinea is the first of two waits, not half of one, and the calendar behaves accordingly.
The half with a published price
The British stage is the predictable one. The Legalisation Office charges £45 / document by standard post, or £40 / document on the next-day route open to registered businesses, and it works entirely by post with no counter to visit in Southampton or elsewhere. The charge is counted on each sheet. Because the figure and the method are both published, this is the part of the road you can put in a spreadsheet with any confidence.
The half you cannot schedule
The consular stage has its own charge and its own rhythm, both set by the desk that gives the mark rather than by any office here. That is simply what happens when a road leaves British jurisdiction partway along. The practical answer is to start early, keep the sheet moving, and avoid the false economy of sending a document you are not certain was the one asked for. A repeat run means both waits again, in the same order.