I don't think that the judge is "offering" the option of the jury returning a verdict on a charge of manslaughter. That option was always there according to the in your post (see "Alternative Counts". IMO, the judge was informing (or clarifying to) the jury that a verdict of manslaughter was possible.
"The prosecution SHOULD decide in advance of a murder trial whether or not an alternative count of manslaughter should be added to the indictment"
It says should, not MUST!
https://www.cps.gov.uk/legal-guidance
/homicide-murder-and-manslaughter