I just found out that the UK issues an entry form at ports of entry for holders of non-EEA travel documents that cannot be endorsed, called IS 116.
What does this form look like? Couldn't find any images on the web.
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This is the front of the form (obtained through a FOI request to the Home Office).
The leave conditions are stated on the rear side of the form, with the stamp being placed at the bottom right corner of the rear side.