One major reason was that the German navy began to operate a campaign against shipping to Britain. This used both surface raiders, including converted merchant ships, and submarines. Cruisers operated in 1914, mainly in the Indian Ocean and Pacific. Three converted merchant ships, the Mowe, Seeadler and Wolf, operated in 1915-1917. Destroyers made several raids against Allied shipping off Holland and Belgium, including a raid on the straits of Dover.

Submarines operated against shipping from 1914, with the first British ship, the SS Giltra, being sunk on the 20th October. For the most part, these followed the restrictive Prize Rules, which required the submarine to warn its target, and to place the crew in a position of safety. Despite this, they were incredibly successful - the RN had very few workable countermeasures. By the end of 1915, 1.1 million tons of merchant shipping had been sunk, mainly by U-boats. There was a bit of a lull in submarine activity throughout 1916, as they were operating in support of the German fleet in the North Sea. Even then, 1.2 million tons of merchant shipping were sunk in 1916. They came back with a vengeance in 1917, aided by the decision to move to unrestricted submarine warfare, where submarines were not governed by Prize Rules. In April 1917 alone, just over half a million tons of shipping were sunk. While the introduction of the convoy system in May 1917 did cut down on the number of sinkings, the number of U-boats committed, and the difficulty of sinking them, meant that sinkings were still in the region of 200,000 tons/month through to the end of the war. By the end of the war, a total of 2,479 ships, with a g.r.t. of 7.8 million tons, had been sunk. The German campaign also engaged the British fishing fleet, with 675 fishing vessels being sunk over the course of the war.

As Britain could not produce enough to feed its whole population, it needed to import foodstuffs. As the UK is an island nation, this has to be shipped in from elsewhere. When the Merchant Navy has taken such heavy losses, the amount of food entering the country sees a commensurate reduction. This makes food rarer, and more expensive. This is also seen in the Second World War, where the Germans attempted a similar submarine blockade of the British Isles.

Source:

History of the Great War - The Merchant Navy, Archibald Hurd, John Murray, 1924