That's not true unless they are smaller shops. For example, because Sainsbury and Tesco now have several 'local' stores, these can stay open for longer because they are just about small enough to qualify as convenience stores. Standard supermarkets and high street stores must stick to six hours on a Sunday. However I did notice the last time I went that most of the shops opened until 6pm on Sundays - but that meant they didn't open until 12.