If you/anyone you know is classed as extremely vulnerable with regards to covid19 go to the .gov website and register ( sorry cant seem to paste link). However, Sainsbury's has a link off their webite:
"To get access to Sainsbury's home delivery slots in the coming days, please register as extremely vulnerable to coronavirus on GOV.UK. If you know it, it’s useful to have your NHS number to hand before you start the registration process. "
I don't know if other supermarkets are doing same.
The form on .gov looks easy to complete but the list of conditions is obviously quite small and doesn't include anyone with any kind of mental incapacity