Unti 1956, Britain believed herself to be a strong country. Why did that change in 1956?
1956 was the year of the Suez Crisis. This crisis was perhaps the final nail in the coffin of Britain's ability to view itself as a major world power.
Up through World War II, Britain had been a major world power at least since the 1600s. Britain had had the most powerful navy in the world and the largest empire. This long history of power had made the British feel that their country was naturally destined to be a major power in the world.
After WWII, this feeling started to decline. It became clear that the United States was the major military and political/economic power in the world. Britain then lost India to independence in 1947, thus taking away the "jewel in the crown" of the British Empire.
But 1956 was the last straw. In 1956, the British (along with the French and Israelis) invaded Egypt when Egypt nationalized the Suez Canal. Britain clearly thought this was an important and necessary action. Even so, the United States disapproved of the invasion. The US then forced the British and others to withdraw.
In 1956, then, Britain had to give up on something it really thought was important because of pressure from the US. This made it clear that Britain was no longer a first-rate world power.