In fact, Britain and every other nation in the war were forced to ration supplies as World War II, as was World War I, was a war of total mobilization during which the economies of all countries involved were geared toward prosecution of the war. In the United States, which was not subject to German blockades, appliances and automobiles were not manufactured during the war years; and pennies in 1943 were made of steel rather than copper. In Britain, housewives were encouraged to save every scrap of textile, rubber, even kitchen grease to be converted to war materiel. Items such as gasoline, heating oils, meat, sugar, and dairy products were strictly rationed to make sure that troops were adequately provisioned. In this regard, Britain's experience was no different from any country on either side.
The reason for this was the Britain was in the middle of a war that made it hard to get supplies and which caused a huge demand for what supplies could be gotten.
During the war, huge amounts of resources were needed to support the war effort. Soldiers needed to be fed and clothed. They needed to have huge quantities of weapons for use against the enemy. This ate up a great deal of what Britain had.
In addition, it was hard for Britain to get more supplies. German submarines, in particular, made it difficult to bring supplies from places like India or the USA.
Because Britain needed large amounts of supplies and because it was hard to bring those supplies to Britain, rationing was necessary.