Why one mark is ever enough
A stamp is worth exactly what somebody else has agreed to read into it. Where two states have joined the same convention, each has accepted the other's certificate in advance, sight unseen, and one mark closes the matter. Outside that arrangement no such promise exists, so trust has to be built a link at a time: a British office speaks for the British paper, and a desk answering to the destination speaks for the destination. The chain is longer because nothing was settled ahead of time.
Where Benin stands
Benin has not joined the Hague Apostille Convention, so papers meant for use there need embassy or consular legalisation instead. There is no agreed reading for an apostille from United Kingdom to Benin, and no apostille issued in Southampton for use in Benin will stand in for the missing agreement. In practice document legalisation for Benin means certification by the FCDO Legalisation Office, then a consular mark added afterwards, in that order and on the same sheet.
What the second desk is really for
The consular mark is the destination's own act, carried out on British soil by people who answer to Benin rather than to Britain. That is the whole reason it cannot be bought from any office here, however senior. It also explains the order: the desk adding the final mark is looking at the British certification already on the page, so the sheet must reach it complete. Sending the paper the other way round produces a return trip and no progress.
Where the money and the waiting go
One half of this road publishes its price and the other does not. The British certification is £45 / document by standard post, or £40 / document on the next-day route open to registered businesses, counted on each sheet rather than on the envelope. The consular stage carries a charge as well, set by the desk that gives the mark. Timing works the same way: the first stage runs at postal speed, and the second begins only once the first is finished.