I recently read that Germany's submarines had a larger effect and were closer to cutting off supplies during World War 1 than during WW2. That got me wondering about how close exactly they were during either war to forcing either capitulation or very serious disruptions in Britain's economy. I know getting enough oil was a big factor during WWI, were there any other commodities that faced similar bottlenecks?
The convoy system was of course very useful in protecting the most important shipping, was it implemented because they feared isolation as a realistic possibility?
Depending on your definition, the U.K. actually did have food and fuel shortages in WWII. This is not to say that they were at risk of starvation, as there was actually an increase in nutritional health, but certainly there was strict and extensive rationing and government control introduced, and the economy was indeed seriously disrupted into the 1950s. Just a few examples: At first, petrol/gasoline was rationed; in 1942, nonessential use of fuel such as in private cars was no longer allowed at all.
Eggs, fats, sugar, meat and milk products were rationed; bread rationing was avoided only by creating a standard whole-grain "National Loaf", replacing all other breads, which was intentionally sold stale so it could be cut into thinner-sliced portions. People were encouraged to eat more locally grown root vegetables (instead of chocolate fudge, why not try toffee carrots, carrot gelatin, or raw carrot jam with cocoa!) and use more space-efficient imported goods such as dried eggs and canned fish.
Agriculture was closely controlled by the government; local committees (War Agricultural Executive Committees) were allowed to set production quotas, mandate planting of needed crops in specific plots of land, and even confiscate farms for the government if necessary. These agricultural reforms were successful in boosting production, but as a combination of war debts and shipping food to even poorer countries in Europe, some food rations remained until 1955.