The copy is the document, not a substitute for it
Families often assume the marked sheet has to be the certificate handed over after the death, and treat parting with it as the price of getting anything done abroad. It is not. The register in Cardiff still holds the entry, and a copy printed from it today carries a registrar's signature exactly like the one printed then. Both are the record; neither is a duplicate of the other.
That changes the shape of the errand. Instead of one irreplaceable sheet making a nervous journey through several post rooms, you send something that can be reordered for the price of another copy. Grief is a bad time to be chasing a lost certificate, and this is the one part of the process that removes that risk entirely.
Two offices, and only one of them is in Cardiff
Cardiff Register Office at City Hall, Cathays Park, Cardiff CF10 3ND issues the copy. The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX puts the mark on it. Neither does the other's job, and the order runs one way: a sheet with no registrar's signature on it has nothing for the second office to stand behind, so it comes back unmarked.
Nothing else is needed before the marking. The signature on a register copy is one the marking office already recognises, which is why the route to apostille a Death Certificate in Cardiff is shorter than the route for a letter or a private paper. Those often need work done to them first; a register copy arrives ready.
Charged by the sheet, handled by post
Two services run: the standard postal one at £45 per document, and a next-day one at £40 per document open to registered businesses. Electronic apostilles are issued in Britain as well. Everything moves by post, so the wait is a postal round trip plus the office's own time, and there is no appointment anywhere in the sequence to book or miss.
The per-sheet charge is worth thinking about before you order copies rather than after. If a bank, a notary abroad and a pension body each need one, that is three copies from the register and three marking charges, and it is far cheaper to discover this at the start than to run the whole loop a second time in a fortnight.
What the mark settles, and what it leaves open
The mark says the registrar's signature is genuine. It does not put the record into another language, and it does not speak for the wording of the entry itself. Where a foreign office works in its own language, the translation is a separate job arranged separately, and it is normally done once both the record and the mark are in place so that the whole bundle is covered.
Nor does the mark decide anything about the estate. It is a certification of provenance placed on a copy, which is why it is quick and impersonal to apostille a Death Certificate in the United Kingdom: no one reads the entry, judges the claim, or asks who the executor is. That reading happens later, in the country where the money or the property sits.