A single stamp, and why that is enough
The apostille lets two treaty members accept each other's documents without checking the same signature twice. Britain and Slovenia both belong to that arrangement, so a stamped British certificate is proved the moment it arrives. Nothing further is added to it, and no consular fee falls due along the way.
Slovenia's place in the Convention
Slovenia joined the Hague Convention in 1991. Since then a British apostille has been the correct and sufficient certification for a document used there, with no consular stamp on top. Saying that plainly matters, because general advice about legalisation usually describes a second stage that has no role at all on this corridor.
The EU rule, and what it leaves out
The European arrangement is often assumed to have swept the stamp away. Regulation 2016/1191 does spare certain public documents any legalisation when they travel between EU member states, but a British document is not covered by it. The exemption misses a paper leaving Sheffield, so the apostille remains the route.
The British stage is postal and central in any case. One office covers the country, so there is no counter in Sheffield to attend and nothing to book. The local task is producing the source paper: the Register Office at the Town Hall for civil entries, a university for a degree or transcript.
Slovene, and what the papers are for
Slovenian public offices work in Slovene, so a stamped English certificate usually arrives with a translation attached. Have those words prepared after the apostille is applied. A translation made beforehand covers the plain document rather than the stamped file a clerk in Ljubljana will be holding when the application is read.
Residence, study and marriage account for most of this corridor. Birth and marriage entries support registration and family status, while degrees and transcripts travel for university places and skilled work. Police records join work applications where a role calls for them, and company records appear where a Sheffield firm registers a Slovenian branch.