I had a response back from CAMRA. They referred me to the amendments to the Town and Country Planning (General Permitted Development Order) which came into force in May 2017 and which removed permitted development rights that previously allowed the demolition/change of use of pubs. They also referred me to Lambeth policy ED8 (public houses) (page 65 of this huge document if anyone's interested:
https://www.lambeth.gov.uk/sites/default/files/pl-lambeth-local-plan-2015.pdf) which sets out the steps a developer has to go through if they want to apply for change of use. Pretty difficult these days.
If we want to apply to register C&S as an Asset of Community Value then it needs 20 of us (residents of Lambeth) to put our names to it and we can do it directly through Lambeth's planning website. Although ACV listing remains a material consideration in planning applications, CAMRA don't think it is as important as it used to be with the new legislation and current Lambeth policy. Pity this legislation wasn't around when they came for the Grosvenor.
Hopefully the C&S will be with us for a while yet...