Since the Assizes of Bread and Ale in 1266, there has been incentive for the commercial bakers to add cheap ingredients to their flour to maximize profits. This law set the price of bread against the weight of a loaf. This was the first English law to regulate food use. Some ingredients are well documented and in some cases proven. There are also many claims of additives that are speculative and unsubstantiated. Here are some cases of additives that are known to have been used.
Chalk powder - This was relatively easy to produce at minimal cost. When added to bread it would increase the weight of a loaf and be harmless to the eater. Flour naturally contains calcium carbonate and increasing the amount had little effect on the bread, provided that it was not done in excess. Thuis sharp practice has been known since Medaeval times.
Potato flour - Another easy way of increasing the weight of a loaf was to boil and sieve potatoes to produce a fine flour. This was a very cheap way of making the dough go further. Potato added to bread flour was encouraged during the two World Wars and people expected such bread, but at other times this practice was used to enhance profit.
Allum - This is a mordent used in the fabric and dying industry. When added to white flour in small quantities it acts as a bleach, making the bread white and improving the firmness of the bread. The quantities have to be regulated carefully to prevent baking the bread too dry. 4 ounces of allum per sac of flour would be the maximum used. (1 sac is 240 pounds or 5 bushels, a bushel being 48 pounds) The allum was dissolved in boiling water and cooled in order to add the yeast and salt. This practice was used in the 17th and 18th century and documented into early Victorian times. As a known health hazard, adding allum to bread is now illegal.
The Corn Laws of 1815 also had an impact on bread production. These statutes set the price of grain in Britain and ensured that the price of imported grain was higher than that of home produced grain. The inflation of grain prices caused riots in the streets of London. The law was repealed in 1846.
The Bread Act of 1822 specified that loaves should be sold by the pound or multiple thereof. As bread tends to dry out and lose weight after baking, the trade would avoid legal action for the sale of underweight bread by introducing the Bakers Dozen - 13 loaves for 12. That way, the weight loss is accounted for in the extra loaf and prosecution for underweight sales is not possible.