One mark, and why it is not enough
Iran has not joined the Hague Apostille Convention. Membership is what allows a single certificate to be taken at face value between two countries, and without it a receiving office has no agreed reason to trust a British mark standing by itself. The paper is therefore certified in Britain first, and a consular stamp is added afterwards on Iran's behalf.
A chain of recognitions
Each stage on this corridor recognises the stage below it rather than the document itself. A registrar signs a certified copy of an entry. A national office confirms that the signature belongs to a registrar, because it holds specimens of the signatures it certifies. Consular staff then confirm that the national office is what it purports to be.
Read in that order the sequence explains itself. No mark on the sheet is an opinion about whether a marriage was happy or a company solvent; each one is a statement about the signature or seal immediately beneath it. That is also why the stages cannot be reordered, run together, or bought separately from different places.
Keeping the finished set intact
Once the last stamp is dry the papers form a single object, even where they look like several loose leaves. Detaching a page to photocopy it, or filing the certifications apart from the sheet they refer to, breaks the very chain a receiving clerk is trying to follow. Forward the set as it came back, and copy the whole of it rather than any one page.
What the British half costs
The British stage is priced per document: £45 each by standard post, or £40 each on the next-day service open to registered businesses. Four sheets are therefore four charges, which is worth knowing before a whole family's papers are gathered up. A consular charge applies on top, decided a long way from Britain, and putting a figure on it from this side would be guesswork.