Why one certificate will not do
Nigeria has not joined the Hague Apostille Convention, and it is membership of that treaty which lets a single certificate be accepted across a border without further endorsement. Documents heading there need embassy or consular legalisation instead: a British certification stage first, carried out by The Legalisation Office (Foreign, Commonwealth & Development Office), then a consular stamp.
Both stages are British errands
It surprises people that a document destined for another continent is prepared without ever going there. The national certification arrives and returns by post inside the country. The consular stamp is added by staff working here on Nigeria's behalf, which is what makes the endorsement meaningful at the far end without the paper having to make the journey twice.
That shapes how the job is planned. There is no international postage to arrange in the middle, no customs step to allow for, and no window in which the document sits in a foreign sorting office. The waits are British waits, and the one crossing happens at the end, in a bag or a courier envelope, with the file complete.
The order that cannot be swapped
The sequence is fixed because each stage looks at the one before it. A registrar signs; the national office certifies that the signature is a registrar's; consular staff endorse that certification. Presenting a sheet at the second stage before the first is done wastes a trip, and no amount of paying for speed at one stage moves the other.
What the first stage costs
The British charge is £45 per document by standard post, or £40 per document on the next-day service open to registered businesses, and it applies to every sheet rather than to the packet as a whole. A consular charge comes afterwards, set outside Britain, so the sensible approach is to fix the number of documents early and let both stages be sized by that count.