The treaty system and the older one
Where two countries have signed the same treaty, one slip proves a signature and neither side asks again. Where they have not, the receiving state uses the older method and puts the file before its own consular officer. Sudan falls into the second group, so the British mark opens the file rather than closing it.
Sudan and the Hague Convention
Sudan has not joined the Hague Apostille Convention. A British paper heading to Khartoum, Omdurman or Port Sudan requires embassy legalisation on top of the British stamp. The United Kingdom is inside the treaty, which is why an apostille can be issued here, but that membership does nothing for a destination outside it.
The Sheffield stage, step by step
Nothing in the British stage is done in person. One office handles the country by post, so Sheffield has no legalisation counter and never did. What the city supplies is the source paper: the Register Office at the Town Hall for civil entries, a court for an order, a university or professional body for anything about qualifications.
An apostille from United Kingdom to Sudan is usually one document among several. Family reunion files list a set of certificates, and posting them in a single round keeps the British marks dated together. That matters at the consular desk, where a complete bundle is handled once rather than opened repeatedly.
Arabic, and the shape of a family file
Arabic is the language of official record in Sudan, so a stamped English certificate normally travels with an Arabic version bound in behind it. Prepare those words after the last mark is applied. A translation made too early describes the plain sheet rather than the stamped bundle that finally reaches the office.
This corridor is driven by families rather than firms. Birth and marriage entries prove relationships for reunion and nationality claims, and police records join them for residence applications. Because such cases can pause and restart, ordering more than one certified copy at the outset is the cheapest insurance a Sheffield applicant can buy.