Nothing is wrong with your document
People reach this page half expecting to be told their paper is defective. It is not. The refusal sits one level above the document, in the arrangement between states that lets a single certificate stand for a whole chain of stamps. Ethiopia has not joined that arrangement, so the shortcut is closed to every British paper alike, however well prepared yours may be.
What Ethiopia asks for instead
Documents intended for Ethiopia require embassy legalisation, which means the Ethiopian side satisfies itself directly instead of trusting a British certificate to speak for the file. More hands touch the paper and each leaves its own mark. The document does not change shape along the way; what changes is the number of offices that have to be content with it before it counts.
Starting the longer route well
The one thing worth doing early is producing a paper that is fit to travel. Sunderland Registration Service issues certified copies of births, marriages and deaths recorded in the city, and such a copy carries the registrar's signature exactly as the entry in your drawer does. Use the certified copy, keep the entry you cannot replace, and the length of the route stops being a risk to anything precious.