The rule about 'gatherings' not exceeding 30 people only applies where people "are present together in the same place in order to engage in any form of social interaction with each other, or to undertake any other activity with each other".
Shoppers in a supermarket aren't there to 'engage in social interaction', nor are they present 'to undertake an activity with each other'. So there is no fixed limit upon how many people can be in a supermarket at any one time.
If you've got a smartphone, you can check how crowded your local supermarkets are at any time by using the Crowdless app, which has been developed by the University of Oxford and is widely praised:
https://www.bbc.co.uk/news/uk-england-oxfordshire-52446658
https://crowdlessapp.co/
The best time to shop in many supermarkets though is often during the last hour of trading. When I visited Morrison's last week at 9pm there were only 3 or 4 other customers present in what is one of their largest stores in the country. Apart from the checkout assistant, I didn't come closer than five metres to anyone else in the store at all.