Apostille vs. Embassy legalisation for Madagascar
Madagascar has not joined the Hague Apostille Convention, so a single British mark does not settle a paper used there. The road runs through consular legalisation instead, which means the file is opened, checked, marked and passed on by more than one body. None of that is unusual; it is simply the older method the treaty was written to replace. What it changes for you is exposure, because the more desks a sheet crosses, the more the choice of sheet matters.
Madagascar's Hague status & what it means
A treaty road ends the argument early. One certificate is issued, and every other member state has agreed beforehand to read it. Outside that arrangement, each office vouches only for what it can see, so the chain has to be built link by link. The practical result is that no stage can be skipped and none can be taken out of order, and an apostille for Madagascar has no place anywhere in the sequence.
The Liverpool → Madagascar route step by step
Liverpool Register Office keeps the city's civil register and can issue a fresh certified copy of a birth, marriage or death entry, and that is the sheet which should do the travelling. The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX then handles the British stage on behalf of the whole country, entirely by post. A consular act follows it, and only after that act is the file actually complete.
Cost & timing for this route
The British figures are published and fixed: £45 / document by standard post, or £40 / document on the next-day route open to registered businesses. Both are counted per sheet, so a second copy doubles that line of the budget. Set against repeating the entire journey, a spare copy is cheap insurance. Time is the part nobody can pin down, since each stage queues on its own schedule and they do not add up neatly.