It would appear that this confusion has been caused by a staggering example of incompetence!
From what I've been told, the intention was to move the HTM guidance documents from the .gov website, to the england.nhs.uk website. Some plum must have spent ages putting a "Withdrawn" watermark on every document, when all that was needed was a redirect of the link activated when the document image was clicked, or just the banner that appears above the documents on the .gov website stating that the guidance has been relocated.
The HTM documents on the .gov website and the .nhs.uk website are identical with the exception of the "Withdrawn" watermark (yes I am sad enough to have done a "compare documents" check).
Whilst there are plans to update FIrecode in the future, as yet they are just plans. To my knowledge there is no iminent update to to the Firecode suite of guidance documents.
Worryingly, I've been made aware that this stupidity has already resulted in two design teams attempting to apply inappropriate design guidance to healthcare premises in the belief that Firecode is no longer valid. Let's hope that these are the only two!