A British food shop sells food products that English people eat that is often not available in countries outside Great Britain. This can include specific brands of tea, chocolate or ready made meals (such as Fray Bentos) as well as products such as Marmite (a spread made from yeast extract) that are specific to Britain and that few other people in the world consume.
Some shops operate online and will send parcels of chosen products to expats located anywhere in the world. Others offer experienced personal shoppers that are familiar with British tastes and can put together hampers of specific foods and household products. Sometimes these stores operate outside of the UK, such as in America or South Africa, so that British people living in these countries can shop locally for their desired products. Often these stores are owned by British people that have an understanding of the specific foods and brands that their countrymen prefer.