Only some shops are allowed by law to open on Sundays and at Easter or at Christmas, and fines exist to enforce these laws.
The law says that on a Sunday:
Some large shops are exempt, e.g. pharmacies, motorway services, restaurants and businesses providing services. There are no regulations for street markets and car boot sales.
The Sunday Trading Act 1994 regulates shop opening hours, not what goods can be sold. However, there are separate laws that cover the sale of certain goods, such as alcohol.
The law says that:
Some large businesses are exempt e.g. pharmacies, motorway services, restaurants, and businesses providing services.
There is a fine of £50,000 for breaking the Sunday Trading Act 1994 or the Christmas Day (Trading) Act 2004.