Why one mark is not enough here
Nepal has not joined the Hague Apostille Convention, so a British certificate alone does not answer for a paper used there. Documents heading that way need embassy or consular legalisation instead. No apostille issued in Bristol for use in Nepal completes a file by itself, and a consular desk supplies the mark that finally does close it.
A signature a stranger cannot test
Start from the reader's side of it. Somebody abroad is handed a British sheet with a name, a seal and a signature on it. They have no way of knowing whether that person could sign such a thing, or whether the seal belongs to a real office at all. Every stage of the route exists to close that gap, one certificate at a time.
An apostille from United Kingdom to Nepal is the first of those certificates. It does not restate what your document says, and it offers no view on whether an entry is right. It answers a narrower question, and answering it narrowly is what makes it useful: a small checkable claim is worth more abroad than a broad one nobody can test.
Where the signing comes from
That is why the source of the sheet matters so much. Bristol Register Office at The Old Council House, Corn Street, Bristol BS1 1JG issues certified copies of birth, marriage and death entries, signed by a registration officer, and a copy ordered there also keeps your own original at home. The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX then certifies that signing by post.
The order is not decorative. Each certificate rests on the one below it, so a sheet whose signing cannot be placed stops the run before it has begun. Getting the document from a body that signs in a recognised way is the real first task, and it is a decision made entirely at your own table.
What it costs to build the run
The British stage is £45 / document by standard post, or £40 / document next-day where the sender is a registered business, and an official charge follows at the consular desk. The money buys certificates about signings rather than any opinion on your paper. That is worth holding in mind when you weigh whether a second copy is worth putting through.