my understanding is you can load your trolly but not pay until official opening.
I could be wrong.
Yes, it's a legally grey area that supermarkets that are complete dumbasses have used for years and generally shy away from without putting things in place and limiting to shorter time periods. The supermarket I used to work in did it unregulated. That's why I know it's a spectacularly stupid idea. It is. They only did it for half an hour and it got banned by one store manager as it caused so many problems.
It leads to bad temperedness, it leads to huge queues, trolley fights, aggression, daft goings on in the car parks, it interferes with stocking up, it's often before shop floor or checkout staff arrive which leads to queries that often can't be answered.
I also have my doubts about what NHS ID will be accepted. A lot of pharmacy staff have smart cards despite being nothing to do with the NHS so you may get community pharmacists and all and sundry sneaking in.
Drastic Dave's misguided. He doesn't have to deal with it, it'll be the staff. He needs to get onto Boris and Rishi and ask for restrictions on deliveries and an hour's extra opening. Extra hour opening and put things in place, it might work. Browsing idea. Very bad idea indeed.