Two roads out of the United Kingdom
British papers leave the country by one of two roads. On the short one a single office adds one certificate and the job is over. On the long one the sheet collects marks at more than one desk before anybody abroad will look at it. An apostille from United Kingdom to Gambia is not an available road, so this corridor uses the long one from the start.
Where Gambia stands
Gambia has not joined the Hague Apostille Convention. Because of that, papers intended for Gambia need embassy or consular legalisation instead, and there is no apostille issued in Belfast for use in Gambia that a desk there will take. This is a rule about the country rather than about your document, and no office on either side has the discretion to bend it.
What a guess costs you
The cheap version of this mistake is the fee: £45 for each document by standard post, or £40 for each on the next-day service open to registered businesses, spent on a certificate nobody will read. The expensive version is the calendar. Every day given to the wrong road is a day the right one has not begun, and deadlines rarely move to suit.
Volume makes it sharper still. Because the charge is counted per document, a family sending four records down the wrong road pays four times over. Working out which papers were genuinely asked for, before anything is posted, is the cheapest single step in the whole business.
Where a Belfast paper comes from
Northern Ireland keeps its own civil register, so a birth, marriage or death recorded here is issued by the General Register Office for Northern Ireland at Colby House, Stranmillis Court, Belfast BT9 5RR, and not by an office in Great Britain. A fresh certified copy from there is a new original, and using one keeps the record you already hold well clear of a long chain of desks.
The office people mean when they say apostille
The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX issues apostilles for the whole of the United Kingdom, and it works by post — there is no counter in Belfast, and none anywhere else. Anyone reading up on what you need for an apostille for Gambia arrives at that office sooner or later. It is the right office for a great many countries and the wrong road for this one.