Apostille vs. Embassy legalisation for Niger
Niger has not joined the Hague Apostille Convention, so a single British certificate does not close a paper used there. Files bound that way need embassy or consular legalisation instead, which is a run of separate acts. No apostille issued in Glasgow for use in Niger finishes the job alone, and the consular desk supplies the act that does.
Niger's Hague status & what it means
Being outside that treaty does not make the British form any more flexible. It stays the same short certificate whatever happens next, and the extra work simply lands on a further desk. So the question of what a form can say, and what it cannot, matters on this corridor exactly as much as it would on a shorter one.
What the form holds, and what it leaves out
A certificate of this kind names the paper beneath it, the person who signed that paper, the capacity they signed in, and the body whose seal appears. It is issued by the FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX, which works by post. Everything on it concerns the signature, and none of it concerns you.
That leaves a long list of things it cannot do. It cannot explain a name change, flag an urgent case, quote a reference somebody abroad gave you, or point out which paragraph matters. Those belong to the sheet underneath, and Glasgow Registration Office at 45 John Street, Glasgow G1 1JE is where a Scottish civil entry can still be reprinted in the right form.
Cost & timing for this route
The British figures are £45 / document by standard post and £40 / document next-day for registered businesses, counted per document rather than per envelope, with an official charge at the consular stage after that. An apostille from United Kingdom to Niger buys one fixed form and nothing else, so paying twice for the same silence is a real risk when the sheet underneath is wrong.
Say it on the paper or not at all
Work on the assumption that the bundle will be read without you present and without a chance to ask you anything. Whatever the reader has to know must be visible on the sheets themselves. That is a strict test, and it is the one worth applying on the morning the extract arrives, while changing it still costs nothing much.