Why the road is long enough to matter
Iran has not joined the Hague Apostille Convention, so a British mark alone does not answer for a paper used there. Documents bound for it need embassy or consular legalisation instead: certification by the FCDO Legalisation Office, then a consular stamp after it. Two stages rather than one, and both of them repeat in full for every sheet you add to the pile.
Where Iran stands
An apostille is not accepted for Iran, which settles the shape of the run before any counting begins. It is worth fixing that early, because the length of the road is what makes the count expensive. A short road forgives a forgotten paper. A road with two stages and two waits does not, and the person absorbing that is you.
How the counting actually works
The published British charge attaches to a document, not to a packet or a visit. That reads as obvious and is missed constantly in practice, because people picture one envelope and imagine one price. Four certificates inside that envelope are four charges, and nothing about posting them together alters the arithmetic at either end of the run.
So the useful work all happens before anything is sent. Draw up the whole list, decide which items are genuinely wanted, and get the copies for all of them at the same time. A list built properly at the outset is the single biggest lever you have over what this run costs and how long it takes to finish.
Cost and timing for the whole set
Because the published figure is per document — £45 standard postal, £40 next-day for registered businesses — the count you fix at the start is the count you pay for. Timing works the same way in reverse: one round trip serves the whole set, while a paper added later earns its own round trip and its own wait at the consular desk.