Nutritionally, rationing was a boon for Britain's poor. Average caloric consumption increased, and the gap between wealthy and poor closed considerably in terms of what was on one's plate, for obvious reasons. Public Health greatly improved as well. In fact, it was the healthiest the populace has been, before or since.

Now, the material conditions are something else, especially if we're wanting to talk about long term effects. That has the effects of rationing tangled irretrievably with the war as a whole, the establishment of the welfare state and NHS, and the postwar economic circumstances. I would argue that the welfare state and NHS did more in the long term for Britain's poor than the rationing system.

However, it was due to the pre-war census conducted in order to calculate food demand that the poverty in the nation was brought to light, and the war effort necessitated uplifting these people to join the military and workforce.

For the prewar situation, I'm mostly referring to The Road to Wigan Pier, by Orwell, and excerpts from The Taste of War, by E. Collingham. The rationing system is also well covered by the latter, with notes from Eggs or Anarchy, by W. Sitwell being useful for insight into the life of the rationing system's architect, Lord Woolton.