I will attempt to answer this question as well as I possibly can. To understand rationing in the late 1930's and 40's, you have to understand that in Britain at that time, there were no greater fears than both being invaded by the Germans, and being the victim of a naval blockade. So essentially, these rationing laws were put in place so that in the event of an invasion or having trade routes cut off, the English people would be able to hold off on nationally farmed goods and their stockpiles of food. In the early 1940's and late 30's, when the threat of invasion was very real, the rationing was just as harsh as it needed to be. At the time, Britain was also looking at a possibility of needing to invoke conscription soon, so, they needed strong and able bodied soldiers, not a race of overweight, lazy citizens. And so, that is why they rationed some foods appropriately as well. What I am saying is, it wasn't what you'd call a reaction to a lack of food, it was more of a preventative measure.

Source:http://www.history.ac.uk/ihr/Focus/War/londonRation.html

best thing I could find for the moment. Hope this helps!