The mark is a border formality
Inside Britain a public record does its job on its own. A certified copy of a birth or marriage entry is taken at face value by banks, employers, landlords and courts here, with nothing added to it. The business of stamps and second desks begins only when a paper has to be read by an office abroad that has never seen a British seal.
That is worth saying plainly, because people arrive with a full folder and assume the folder needs treating. It does not. What you need for an apostille for Mauritania starts with a smaller question: which of these sheets is actually going to Mauritania? Everything else can stay exactly as it is, in the drawer, doing its ordinary work.
Where Mauritania stands
An apostille for Mauritania is not the certificate that finishes the job. Mauritania has not joined the Hague Apostille Convention, so the single mark that closes a file for a member country has no standing there. Document legalisation for Mauritania runs through consular offices instead, which puts a second desk after the British one and a second charge beside it.
So an apostille from United Kingdom to Mauritania is not refused through any fault of yours. It simply is not the currency the far end deals in. There is also no apostille issued in Manchester for use in Mauritania waiting behind a local counter, because no counter in the city hands one out.
What the travelling sheets go through
For a civil record, Manchester Register Office issues certified copies from Heron House, 47 Lloyd Street, Manchester M2 5LE, and that copy is the sheet that makes the journey. The Legalisation Office (Foreign, Commonwealth & Development Office) then works on it by post from PO Box 6255, Milton Keynes MK10 1XX, charging £45 for each document by standard post.
The file then moves on to a consular desk, which adds its own mark and sets its own charge. Two desks mean two waits, and neither can be hurried by the person waiting. Because only the exported sheets go through this, the size of the job is set by the choice you made at the start, not by the thickness of your folder.
What stays behind
Nothing you keep at home is weakened by any of this. The register still holds the entry, your own copy still proves what it always proved, and no British office needs to look at it again. Treat the route as something done to one copy, for one purpose, in one direction, and a job that looked like a folder shrinks to a sheet or two.