Why a single mark closes the file
An apostille is a slip confirming that a named official signed the sheet under it. Countries inside the shared treaty agree to accept each other's slips, which spares everyone a chain of embassy stamps. Both countries here are inside it, so a British sheet marked in Milton Keynes needs nothing added before a Spanish desk will read it.
The European shortcut, and why it misses you
People often assume a paper heading into an EU country slips across without any stamp. A European scheme does do that for certain public forms, but it does not stretch to a sheet issued in Britain. The older route applies in full instead, and an apostille from United Kingdom to Spain is the ordinary way a British record is made usable there.
That is worth knowing early, because the assumption costs weeks. Someone who posts a bare birth entry to Valencia and waits usually gets it back unread, having lost the time the mark would have taken. The stamp is neither hard to get nor slow, but it does have to be got before the sheet leaves.
Counting the sheets before anything is posted
The cost lands on each document rather than on the packet, and that one rule shapes the whole job. A file holding a degree, a transcript and a police record carries three charges, not one. Working out what the Spanish reader genuinely asked for is therefore the cheapest hour you will spend on the errand.
Two rates exist and the lower one is open only to registered businesses using the next-day service. A household sending a single certificate pays the standard postal rate instead. Neither rate shifts with the kind of paper, so a short birth entry and a long set of company records cost exactly the same to mark.
What comes back, and how long to allow
The slip returns attached to the sheet it describes, and the two travel together from that point on. A Spanish office reads them as a single item, so keeping the pair intact matters. Where a translation is wanted, have it made after the mark is on, so the translated set matches the bundle that will actually land.
Times are not fixed. The route runs on the post and on a queue, so start earlier than feels needed rather than budgeting to a date nobody has quoted. What you need for an apostille for Spain is mostly patience with the envelope, since no second office has to look at the file once the mark is attached.