Read there since 2001
Namibia joined the Hague Apostille Convention in 2001. From that point a British certificate has been the correct and sufficient answer for a paper used there, and nothing follows it. An apostille issued in Bristol for use in Namibia is complete when it lands back on your mat, with no consulate to approach afterwards and no second mark to buy from anybody.
One office, one act
The FCDO Legalisation Office at PO Box 6255, Milton Keynes MK10 1XX issues every British mark of this kind and takes its work in by post. It looks at the signature and seal on the sheet, satisfies itself they belong to somebody entitled to sign, then attaches its own certificate. An apostille from United Kingdom to Namibia is that single act and nothing more.
Bristol Register Office at The Old Council House, Corn Street, Bristol BS1 1JG owns the earlier job. It prints certified copies of birth, marriage and death entries, and a copy ordered there can travel in place of the original you keep at home. That one habit removes the only serious risk on a route this short, which is a paper lost on the way.
The steps nobody asked for
Because the longer route is the one people hear about, readers often arrive expecting more stages than exist. They set money aside for a consular charge, hunt for an embassy appointment, or hold the file back while they puzzle out an order of steps that does not apply here. All of it costs days, some of it costs money, and none of it improves the sheet.
So keep the route boring. Order the right copy, pay the published charge once for each sheet, wait for the post, and hand the finished pair on. Anything beyond that is extra work bought on the strength of somebody else's story about a country with a different rule.
What it costs, and when
The published figures are £45 / document by standard post and £40 / document next-day, the quicker rate being open to registered businesses. Both are counted per document rather than per envelope. There is no second stage here, so the whole outlay at this end is visible before you begin, which is unusual enough on these routes to be worth saying plainly.