Why two desks, not one
Angola has not joined the Hague Apostille Convention, so a British mark on its own does not answer for a paper used there. Documents heading that way need embassy or consular legalisation instead: certification by the FCDO Legalisation Office, and then a consular stamp added on top of it. Two acts, two bodies, and a gap in between that belongs to nobody.
Where Angola stands
The position is plain enough: an apostille is not accepted for Angola. What you need for an apostille for Angola turns out to be the wrong thing to plan around, since that mark is not read there at all. Getting the status right on day one saves a full postal round trip, and it saves the sinking feeling that comes when a sealed sheet lands back with nowhere to go.
The three handovers
The chain has three joins, and each one is a place where a file can sit for weeks unnoticed. The paper goes to the register office and comes back. It goes to the Legalisation Office and comes back. It goes a third time to the consular desk. Nobody standing at any of those points is watching the other two on your behalf.
That is why the certified copy earns its keep here. A record that has crossed three counters has had three chances to go astray, and a birth, marriage or death certificate rebuilt from nothing costs weeks that were never in anyone's plan. The copy can be replaced; the entry it was made from stays in Nottingham, where it was written.
What you can pin down
One figure on this route is published: £45 for each document by standard post, dropping to £40 for each document on a next-day footing where the sender is a registered business. Both are counted sheet by sheet rather than by envelope. The consular charge and pace are set well away from here and are not yours to fix, so give that end of the run the widest room you can bear.