Two thoughts on this: first, if there is no known address, returning an empty string value in JSON would be better than returning an unpredictable sequence of "XXXX"s.
Second, it is a great shame that the UK national records don’t keep forever images of paper documents submitted. When I say “images” I am thinking of the fact that, in my home of Hong Kong, the Companies Registry used to image and store all submissions on (analogue) microfiche. When the time came to go digital, they digitised all of those microfiches. Some of the documents in those images date back to the 1920s, soon after company registration became compulsory in HK (which was in 1911). The records even survived over 3 years of Japanese occupation during World War II. It is true that the HK registry still has a paywall, which I have been urging them to tear down, and I applaud the UK for having done that. But at least the records exist.
Was there ever a set of UK Companies Registry microfiches, and if so, does a set still exist that might contain images that could be digitised?