Yes, but a lot aren’t enforcing it. Anyone can still go in and a lot of people just say “oh but I’ve got asthma/weak immune system/respiratory problems” and just let them in anyway. Some stores just don’t stop anyone, in practice it’s more advisory.
A large portion of NHS will probably miss the window for this too. I’m not NHS but every shift I finish anywhere between 7 and 10am. Almost no shops have that kinda opening time to allow me to benefit from this if I was NHS.
I work in retail food distribution so I’m communicating with stores a lot at the moment and this is just the feedback I’m getting. Goes without saying, this system has just started so there is going to be issues. Hopefully in the coming days/weeks things will smooth out and everyone can get suitable access to the foods they need. Also every store and chain is different. Stay safe everyone!