The stamp and the paper become one object
An apostille is not a mark across your document. It is a separate certificate, filled in for that one paper and fastened to it, and the fastening is the thing that ties the two together. From then on you are carrying a single item. Treat it as two and you have thrown away the link that made either half worth anything.
What Germany does with the pairing
Germany joined the Hague Apostille Convention in 1966, so an office there knows the certificate on sight and knows what it stands for. It reads the slip, then reads the paper the slip is attached to, and the two must agree. Requests for document legalisation for Germany end at that single certificate, because no consulate has any part to play in this corridor.
Getting the certificate added
One body issues these certificates for the whole country, and it works by post. The route for an apostille from United Kingdom to Germany is therefore short: a paper that an official signed goes in, and the same paper comes back with a certificate joined to it, for £45 on the standard postal service. No stage of it happens in Sunderland itself.
Handling it afterwards
Once the bundle is back, the risk moves from the process to the handling. Loose sheets drift apart in a folder; a helpful colleague scans the pair and forwards the scan; somebody unpins it to make a tidy copy. Each of those turns a certified document into two sheets with no proven relationship, and putting it right means buying the certificate again.