Two ways to prove a signature abroad
A treaty slip works only between states that agreed to honour each other's stamps. Everywhere else, the receiving country sends the file past its own consular officer first. Sri Lanka follows the second route, which is why a British paper needs a mark from each side before an office in Colombo will act on it.
Sri Lanka and the Hague Convention
Sri Lanka has not joined the Hague Apostille Convention. A British paper bound for Colombo, Kandy or Jaffna requires embassy legalisation on top of the British mark. The United Kingdom is a treaty party, which is why an apostille is issued here at all, but that helps only where the destination has signed the same agreement.
The groundwork done in Sheffield
None of the British stage happens locally. One office covers the country by post, and it keeps no public counter in Sheffield or anywhere else. What the city supplies is the source paper: the Register Office at the Town Hall for civil entries, a solicitor for a power of attorney, a university or professional body for a qualification.
An apostille from United Kingdom to Sri Lanka is usually one part of a larger application. Nationality and residence files list several documents at once, so ordering the certified copies together and posting them in a single round keeps the marks aligned and spares you a second wait at the consular desk.
Sinhala, Tamil, and the nationality files
Sri Lanka runs its public business in Sinhala and in Tamil, so the language of the version you send should follow the office that will read it. Prepare those words after the stamps are applied. A translation made earlier covers the sheet you started with rather than the marked bundle that finally arrives.
The corridor is shaped by people with ties to both countries. Dual nationality claims rest on British birth and marriage entries, and residence or retirement files sit beside them with police records attached. Property matters bring powers of attorney into the mix. In each case the same two marks are needed, and the timetable belongs to the second one.