You are looking at the wrong side of the market. You need to look at the market from the perspective of the seller v

For examples, do the 7 bigges supermarkets in britain, Tesco, Lidl, etc, have a monopoly?

I can buy all my food from the snaller shops and market markets, as a buyer.

But if I am a seller, and I need to sell mass market food, and all the big supermarkets refuse to carry it, you will go bankrupt.