One paper, three jobs, three bodies
Most trouble on this route comes from expecting one office to do everything. A register office issues records. A single national office marks the signatures on public papers. Consular staff for the destination add a stamp of their own. Each is a separate body with separate rules, and the paper visits them in a fixed order.
Why the third desk exists
Papua New Guinea has not joined the Hague Apostille Convention. Where a country has joined, the British mark is honoured on its own and the chain stops at two bodies. Here it does not, so an apostille from United Kingdom to Papua New Guinea is followed by a consular stamp before the file counts at the far end.
What each desk will not do for you
The register office will not mark a paper it did not print, and it has no say in what happens after the copy leaves. The marking office does not issue records, correct a spelling or judge whether the contents are true; it looks at the signature and the seal and nothing else.
The consular desk, in turn, is not a shortcut past the first two. It works from a file that already carries the British mark, so approaching it first simply returns the papers. Knowing these boundaries saves the wasted round of post that catches most people out once.
The part that is yours
You choose the version of the paper that travels, and that choice sets up everything else. A fresh certified copy of a civil record keeps your own certificate at home while the copy makes the long trip through three sets of hands, and the register can print another if an envelope goes astray on the way.
After that, what you need for an apostille for Papua New Guinea is patience in the right places: one charge per paper at the British stage, one journey each way through the post, and room in the diary for the consular desk that finishes the job.