Just some old information which already exists in posts to this forum, but since the URI guide I think the following prefixes have appeared in the “Companies House zoo”:
As my post from a long time ago notes sometimes some more hints may appear on the XML forum or by looking at the XML schemas for the XML gateway.
All I know is that documentation is … intermittent. Here are some findings on some of those you list from this forum, from the most recent:
Overseas entities:
Charitable incorporated organisations:
Further education institutes:
Protected cell companies
Perhaps see here?
Scottish Qualifying partnerships:
Companies with letter suffixes
There seems to be nothing on the “why” of these (but note that the Companies House data set is … rather old) but some observations here: