The translation is tied to one sheet
A certified translation for Namibia is not a general rendering of your life events. It is a rendering of one physical sheet, described on the signed page that comes with it. That description is what lets a reader match the two documents, and it is also why swapping the sheet afterwards undoes the match.
Settle the source before anything is written
Where the paper is a civil record, Westminster Register Office at 317 Harrow Road, London W9 3RJ issues certified copies of birth, marriage and death entries. Ordering that copy first, and translating from it, means the English set belongs to the sheet you will hand over. The office works by post and by appointment, and it does not translate anything.
This also keeps your own certificate at home. A certified copy stands in for the original wherever a copy is accepted, so the paper you have kept for years never has to go into an envelope. A certified translation into English in London is built from an image either way, so nothing gains from risking the original in transit.
English is the language this corridor asks for
Papers put to official use in Namibia have to be in English, so that part of the job is settled before you start. An official English translation in London therefore has a clear target and no room for debate about the language, which removes one decision from your list and leaves only the question of which sheet goes in.
Who signs, and what the signature covers
The work is signed by a certified translator, who produces a certificate of accuracy naming the sheet, the date and themselves. No oath is taken and no court holds a register of the person involved. The claim is deliberately narrow: these two documents say the same thing, and here is the person who says so.
Cost, days and doing it once
The rate is about £25 a page, so the bill follows the number of sheets. Standard work runs 2–3 business days and express work 24 hours. Getting a certified translation in London for Namibia in the right order is mainly about paying that rate a single time rather than twice for the same record.