During World War II, as part of the rationing effort by the British Government, all cheeses produced for sale had to be a certain type of Cheddar, which was nicknamed Government Cheddar. This ban of all cheeses except Cheddar lasted until 1954, 9 years after the end of the war. The ban on cheeses had a devastating effect on the number of cheese producers and the variety of cheeses from the United Kingdom.
Great Britain was a heavy importer of food. Because it is an island nation, much of that food came by boat. During World War I, German submarines were used to sink ships bringing food to Great Britain, in an attempt to starve the British into surrender. The government established food rationing so that the nation would not run out of food.
The British government instituted rationing again during the Second World War, but this time earlier in the war than it had during the previous war. Each person had to register with local shops, and was given a coupon book to use at that shop. When someone purchased food at a shop, they had to present a coupon as well as money to purchase the item.
Before the World War I there were about 3,500 independent cheese makers in Great Britain. By 1945 there were less than 100.
The ban had long-lasting consequences. Before the second war, in 1939, 514 farms in southwest England were making different varieties of Cheddar. In 1974, twenty years after the ban was lifted, there were 33 farms making cheddar, most of which was very similar taste and quality.
The revival of Brittish cheeses started happening in the 1990's, especially after the abolishment of the British Milk Marketing Board in 1994. Many cheeses, such as Wensleydale, that had nearly become extinct have made a come back. Still, the variety of British cheese is far from what it used to be.
The term Government Cheddar also refers to cheese made in the United States, mostly after World War II.
You can read more about British war rationing at this Wikipedia article.