The route, before anything else
Guinea has not joined the Hague Apostille Convention, so an apostille for Guinea is not the certificate its offices act on. Papers going there require embassy legalisation instead, through a chain of desks rather than one stamp. That is fixed and there is nothing on the British side that changes it. What you can change is the quality of the paper you feed into that chain, and it is worth more attention than most people give it.
What an old sheet costs you
An entry copied out by hand generations ago is a lovely thing to own and an awkward thing to send abroad. The writing can be hard to read, the layout is unfamiliar, and the paper itself may be soft at the folds. None of that makes the record invalid. It simply means every clerk who handles it spends longer on it, and a file that takes longer is a file with more places to stop.
The fresh copy, and why it travels better
Newcastle Register Office can print a current certified copy of any birth, marriage or death entry it holds. Every name and date on it matches the sheet you already own. The difference is that the sheet is new, legible and, above all, replaceable. Readers searching for what you need for an apostille for Guinea generally think first about stamps and fees, when the first real decision is which physical object they are prepared to lose.
What the British stamping route looks like
The Legalisation Office (Foreign, Commonwealth & Development Office) issues every British apostille and works by post rather than across a counter. An apostille issued in Newcastle upon Tyne for use in Guinea would come from there, at £45 for each document by post or £40 for each document on the next-day service open to registered businesses. Useful to know, and not the route Guinea uses.
A sensible order of work
Order the fresh copy, read it against the old one to be sure the names and dates carried across, then start the chain. Document legalisation for Guinea takes long enough that you do not want to discover a smudged surname halfway along. An apostille from United Kingdom to Guinea would need the same clean start, so the habit is worth keeping whichever country you are writing to next.