There wasn't a lack of food in the UK, not in the sense that people weren't getting enough to eat or were suffering malnutrition. What there was is a lack of variety of food. Anything which was imported (citrus, tropical fruits, tea, coffee, sugar), expensive (meat) or important to the war effort (fats, meat, canned anything) would be rationed. Rationing was also introduced to prevent hoarding, shortages, price gouging and ensure everyone got their fair share.
It has even been claimed that people in the UK were healthier during the war because they were eating a healthier diet prescribed by the Ministry Of Food. The ration cards ensured everyone got enough, and also that few ate to excess. The study assumed everyone "could eat as much potato, vegetables, and wholemeal bread as they wanted". This was a fair assumption, they were not rationed during the war.
The UK had to feed a population of about 50 million during WWII. At the start of the war it imported...
70% of its food; this required 20 million tons of shipping a year. 50% of meat was imported, 70% of cheese and sugar, 80% of fruits, 70% of cereals and fats, 91% of butter. Of this, 1/6th of meat imports, 1/4 of butter imports and 1/2 of cheese imports came from New Zealand alone, a long ways away by shipping lanes.
It's wrong to conclude that Britain could not feed itself. Some of this would be imported as luxuries, others for economic reasons, that it was cheaper to import food than produce it locally. When Britain realized it was going to war, local food production was ramped up. (If anyone has UK agricultural production numbers for 1935-1950 that would be great)
On top of food, Britain imported fuel, raw materials and manufactured goods. Wartime requirements increased these needs dramatically requiring a million tons of imported material per week to stay alive and in the fight. Wartime priorities meant luxurious food items would be skipped.
British shipping was quite vulnerable to attack, and the early loss of France and Norway allowed the Germans to put aircraft and submarines even closer to the UK. The British were losing hundreds of thousands of tons of shipping each month putting further strain on their supply line.
Finally, the wartime needs meant that much of the traditional farming population were needed for the war effort. By war's end 7.6 million people, 15% of the population, served in the British military. Plus more working in factories producing war goods. This left a shortage of farm labour. The Women's Land Army was formed, first as volunteers and later by conscription, to make up the shortfall.
Farmers increased the number of acres under cultivation from 12,000,000 to 18,000,000, and the farm labor force was expanded by a fifth, thanks especially to the Women's Land Army.