Rationing in the UK was implemented in a rather lackadaisical, disorganised fashion. Rationing was not introduced until December 1917, when local schemes began. Rationing wasn't implemented nationally until April-July 1918. That said, measures were taken earlier in the war to ensure that Britain's food supply remained relatively unharmed by the war.

Over the first year of the war, food prices in Britain rose by a total of 32%, and increased a similar amount over the next year. As a result, the Board of Trade implemented a Departmental Committee on Prices in June 1916. In September of that year, it recommended that the government make no major intervention in the food market. However, thanks to major parliamentary pressure, the Royal Commission of Wheat Supplies was set up in October. The next month, the Food Department of the Board of Trade was formed. This had considerable powers under the Defence of the Realm Act (DORA), and its leader received the rather grandiose title of Food Controller. The post went unfilled until December 1916, when Lord Devonport was appointed to the role following the fall of Asquith's government and the accession of Lloyd George. Devonport, unfortunately, took little action, even in the face of the German introduction of unrestricted submarine warfare. This cut into British shipping like never before, and hence cut into British food supplies, and led to major price increases, which Devonport did little to curb. Only in June 1916, when he was replaced by Lord Rhondda, did the government take the needed action.

Rhonnda's main policy was to prioritise bread over all else. The idea was to ensure that the British population at least had bread, so they wouldn't mind the lack of less-staple foodstuffs. Policies implemented included reducing the numbers of sheep and cattle, as feeding grain to animals was less efficient than feeding it to humans. The government also increased the acreage of land devoted to growing grain, and increased the percentage of flour recovered from wheat, as well as allowing dilution of wheat flour by flour from other grains. Campaigns were introduced to discourage wasting food, and to encourage eating less bread. In April 1917, flour mills and wheat importation were brought under the auspices of the Wheat Commission.

Food queues, especially for imported commodities like sugar, began to grow as 1917 progressed. In response to this, a number of local authorities began to introduce their own rationing schemes for such commodities. These schemes were coordinated with the Food Controller, but otherwise were taken at a purely local level. Birmingham was the first city to introduce rationing, giving each person in the city a maximum allowance per person, taking no account of age and physical activity. At first, butter, sugar and margarine were rationed, but the start of 1918 saw much more become rationed. In February 1918, meat became rationed in London and South-East England, with each person receiving 15d worth of meat per week, as well as 4 ounces of bacon or ham. Local schemes also covered jam, cheese, tea and lard. These local schemes were generally quite unfair - people in the cities were most affected, while those in the countryside might not be brought under rationing until the national scheme was introduced. Additionally, because local authorities were allowed to, to a certain extent, set the level of rationing, people other parts of the country might receive more food than you did.

In April 1918, the National Meat Scheme was introduced, extending meat rationing to the whole country. At the start of May, the meat ration was cut to 12d, but the bacon ration increased to 8 ounces. Additionally, offal had been removed from the ration. In July, rationing for butter, lard, tea and sugar was extended nationwide through the introduction of the National Rationing Scheme. Even after this, the local schemes for other commodities continued. Equally important to the rationing was a system of price controls introduced by the Ministry of Food. These controls applied to all staple foods, and most foods eaten by the typical British citizen. They helped ensure that the typical diet saw no serious interruptions, though the poorest in society did suffer somewhat as prices increased.

Source:

The First World War and Working-Class Food Consumption in Britain, Ian Gazeley, Andrew Newell, IZA Discussion Paper No. 5297, November 2010