Comparing a retail business with a manufacturing business is naive - rather like comparing apples with oranges. Companies may have a range of different financial models, whether driven by profit margins, earnings per share, return on capital or cash generation.
Polarising the debate by focusing on one formula is trite. The debate on the extent to which supermarkets squeeze their suppliers has been raging for years and will rage for many more.
In having that debate, however, let's concentrate on evidence that illuminates rather than obscures the subject. If profit margins were the sole financial measure, why are the margins of British retailers higher than those of Continental retailers?