A signature the court stands behind
A sudski tumač is a translator a court has appointed to certify translations, and the appointment is the point. When that person seals a page, they are putting a public standing behind the words, not just a claim of skill. The seal shows a name and the title itself. Anyone reading the Serbian later can trace it back to a real, listed individual, which is exactly what an official reader wants.
The file has to speak Serbian
A certified translation into Serbian in London is required for papers that will be used in Bosnia and Herzegovina, whatever language they were issued in. An English certificate on its own leaves a clerk with nothing to work from. So the Serbian version becomes the working text and your English page travels with it, close enough that a name or a date can be checked across the two in seconds.
What the seal covers, line by line
The seal covers everything the translator wrote, and that includes the parts that are not sentences. A registrar's stamp, a fee mark, a handwritten correction in the margin, a signature crossing two boxes — each is noted in Serbian where it sits on the page. Leaving one out would break the match between the two sheets, and the match is what the sealed set exists to prove.
Price by the page, speed by the day
Cost follows the paper: £25 a page, counted by printed sides. The standard clock runs 2–4 business days and starts when a readable scan arrives, not when you first write in. Express work is done inside 24–48 hours. Notarisation can be added on top where a file calls for it, though the court-appointed seal is what carries the Serbian text by itself.
The sheet you keep
A certified translation in London for Bosnia and Herzegovina asks nothing of your original beyond a good scan. Where the record is a civil one, a fresh certified copy from Westminster Register Office can stand in for the sheet you hold, so the one in your file never has to move. Ordering that copy early also gives the translator a cleaner image to work from than a folded, faded page.