A narrow promise, honestly kept
People expect the slip to bless the whole paper. It does not. It confirms the signature, the seal and the post held by whoever signed, and it stops there. Read that way, an apostille from United Kingdom to Haiti is a short letter of introduction between two states, not a judgment on the words in your file.
Haiti's place outside the Convention
Haiti has not joined the Hague Apostille Convention. That single fact changes the shape of the job rather than the meaning of the mark. The British slip still confirms the same three things, but it no longer travels on treaty terms, so a consular officer for Haiti puts a further stamp on the file before a desk there will act on it.
Everything turns on who signed
The Legalisation Office holds specimen signatures and seals for the bodies that sign public papers here. A registrar, a court officer and a government department are all on that footing. A certified copy from Westminster Register Office is therefore easy to mark, because the signature on it belongs to a post the office already knows.
This is also why your own certificate should stay in a drawer. A fresh certified copy of a birth, marriage or death record has the same standing as the one you keep, so send the certified copy on the long trip and a lost envelope becomes a reprint instead of a loss.
What the consular stage adds
The consular officer is not checking the register again either. They are satisfying themselves that the British mark is genuine and that the file is one their own system will accept. That is the answer to what you need for an apostille for Haiti: a paper the British office can mark, then time for the desk that marks the marker.
Costs and clocks split the same way. The British charge is fixed per paper and published; the consular stage is priced and paced by the receiving side. Plan around the half you do not control, and start early enough that a second round of post does not eat your deadline.