Ah, I didn’t spot the hidden hyperlink!

As to your theory, wieesso, it works up to a point. A number of the foreign registered aircraft referred to in post #5 were previously UK registered, but not all. For example, Aerovan ZK-AWV had only a UK G Class registration (G-21-3) before going to New Zealand and for Gemini HB-EKS there is no record of any registration prior to its Swiss one. Maybe in these cases, the answer to my question is that they were sold with the benefit of a new UK CofA. Maybe they needed to have that for the ferry flight?