Why one stamp settles it
The treaty behind the apostille has a simple purpose: members agree to accept one another's stamps without further argument. Britain and Italy are both inside it, so a British certificate carrying the stamp arrives in Italy already proved. The work that remains is about language and about the office receiving the paper.
Italy's place in the Convention
Italy joined the Hague Convention in 1978. Since then a British apostille has been the correct and sufficient certification for a document used there, with no consular stage on top. That is worth saying clearly, because guidance written for other destinations often assumes a second stamp that has no role at all on this route.
The EU rule, and why it misses British paper
It is easy to assume the European arrangement makes all this unnecessary. Regulation 2016/1191 does remove legalisation for some public documents travelling between EU member states, but a British paper is not covered by it. The exemption does not apply here, so the apostille remains the route from Sheffield to any Italian office.
Nothing about the British stage happens locally either. One office covers the country by post, so there is no legalisation counter in Sheffield. 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 for a degree.
Italian, the comune, and the sworn translation
Italian offices work in Italian, so a stamped English certificate is usually presented with a translation attached. Italy often expects that translation to be sworn before a court, which is a formal act by the translator rather than another government stamp. Have the words prepared after the apostille is on, so they cover the stamped document.
The comune is where most of these files end up. Registering a residence, recording a marriage, or supporting a claim to citizenship by descent all rest on British birth, marriage and death entries. Property purchases bring powers of attorney into the picture. In every case one stamp is enough, and only the Italian paperwork varies.