The chain starts with a signature, not a stamp
People tend to picture the process as a stamp being added to their document. It is closer to the reverse: an official signature already on the sheet is what the British mark points at and stands behind. No signature of that kind, no mark. So the first job is not booking anything, it is holding a version of the paper that a public body has signed.
This is why the same errand can be short or long depending on what you are holding. A certificate that came from a register is ready. A form printed at home, a scan emailed by a school, or a photocopy taken years ago is not, and turning it into something markable is work that happens in Wales, before any envelope goes anywhere.
Why Somalia keeps the older arrangement
Countries can accept foreign papers in one of two ways. They can share a treaty and take a single slip from every other member, or they can keep the older arrangement in which their own officials look at each file. Somalia has not joined the Hague Apostille Convention, so the second way applies and embassy legalisation follows the British stage. No accession year is attached to Somalia either, so this is not a route on the verge of changing.
What happens in Wales
Cardiff Register Office at City Hall, Cathays Park reissues civil records, and each copy carries a registrar's signature of its own. From there the sheet travels to the FCDO Legalisation Office in Milton Keynes, which handles the whole country by post. There is no counter in Cardiff for this stage and nobody to see in person about it.
The charge is fixed per document rather than per envelope, so the size of the bundle is the one real cost decision on this side. Marking a paper that nobody on the Somali side asked to see is money spent twice over, because the consular stage will charge for its own attention to that sheet as well.
The second mark, and who applies it
The consular staff acting for Somalia in Britain add the closing mark. Their stage is different in character from the British one: a person reads the file rather than a clerk matching a signature against a list, and they keep their own record, their own pace and their own list of what they will look at. That is the half of the route that sets your calendar.