The route this country asks for
Sri Lanka has not joined the Hague Apostille Convention, so a British mark by itself does not answer for a paper used there. Documents bound that way need embassy or consular legalisation instead. No apostille issued in Bristol for use in Sri Lanka finishes the job alone, and a consular desk supplies the mark that does close it.
Two things with the same words on them
A register holds an entry. The sheet you carry about is a printout of that entry, certified by the office that made it. The two are easy to muddle because the wording matches, but only one of them can be lost. An apostille from United Kingdom to Sri Lanka is applied to the printout, and the entry behind it sits in Bristol untouched throughout.
That is why a fresh certified copy is the right thing to spend on this route. Bristol Register Office at The Old Council House, Corn Street, Bristol BS1 1JG prints one from the entry it keeps, and can print another if the first goes astray. Sending a sheet you could replace turns the worst outcome from a disaster into an errand.
Papers with no register behind them
Not everything works that way. A court judgment, a degree award or a company record comes from a body with rules of its own, and a second copy may be slow, costly or simply not on offer. Where that is the paper in question, the risk of the journey is real rather than theoretical, and it deserves a decision rather than a shrug.
The practical answer is to keep the number of journeys down. One sheet, chosen carefully and sent once, is safer than several shuttling back and forth. It also means reading the paper closely at the start, because a run begun with the wrong document ends with the wrong document, only later and after more has been spent on it.
What each stage asks for
The British charge is £45 / document by standard post, or £40 / document next-day where the sender is a registered business, and an official charge arises again at the consular stage. Both are counted per sheet. That is one more reason to be sure the sheet is the right one before the first envelope is sealed and posted.