Every desk leaves something on the page
Legalisation works by adding to a document, never by describing it from a distance. Each stage puts a mark on the paper or fastens something to it, because that mark is the whole product. A short road adds one. A longer one adds several, and the sheet arrives back thicker, stamped and fastened in ways you did not choose.
Where Guinea-Bissau stands
Guinea-Bissau has not joined the Hague Apostille Convention, so papers intended for Guinea-Bissau need embassy or consular legalisation instead. There is no apostille issued in Belfast for use in Guinea-Bissau that a desk there will act on. Since that road runs past more than one office, the point about accumulated marks stops being a curiosity and starts being the reason to think hard about which sheet travels.
A certified copy is a new original
Northern Ireland runs its own civil register, and the General Register Office for Northern Ireland at Colby House, Stranmillis Court, Belfast BT9 5RR issues certified copies of birth, marriage and death entries recorded here. A copy from that office is not a photocopy of your certificate — it is a fresh issue from the register itself, with the same standing. It can also be ordered again, which is the property that matters on a long road.
Sorting the pile before anything moves
Papers divide neatly into two groups here. Register entries can be reissued at will, so they are cheap to risk. Everything else — a court order, a company record, a letter signed by one named person — traces back to a single source that may no longer be easy to reach. An apostille from United Kingdom to Guinea-Bissau is not the route in any case, so the whole of the decision is about the paper rather than the stamp.
The office that issues the stamp this country does not use
The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX issues apostilles for the whole of the United Kingdom, working by post with no counter to visit in Belfast or elsewhere. Anyone reading up on what you need for an apostille for Guinea-Bissau meets that office first and then finds it is the wrong turning here. Its charge is still worth knowing, since it sets the floor for what any longer route will run to.