We are currently aware of 25 UK companies which were once visible in the system but seem to have vanished. 5 of them were dissolved before the end of 1993, but the other 20 were all fairly recently dissolved from Mar-2011 onwards, and are not even visible at the URI page (Error Page).
In WebCheck, they also produce a failure message when searched by company number.
I hereby upload a list in case anyone can shed light on what happened to them.