The British empire grew at first on a mostly random basis. Colonies in the Caribbean were acquired to act as sugar plantations and pirate bases against the Spanish. Trade posts in India, meanwhile, were established in order to cut into the spices market in competition with the Dutch. Eventually the conquest of India by the British East India company led to a massive landholding that the British needed to protect. This led to a particular line of British strategic thought in which land acquisitions were made for the express purpose of either protecting India, protecting trade to India, or even to give ports of access to British ships to India. This is why, when India left the empire, the empire fell apart fairly rapidly. With the economic impetus behind the empire gone, there was very little motivation for the British to continue to fight for their landholdings. A high profile example is the Suez Canal crisis of 1955. Once the lifeblood of British imperial trade, with India gone, there was a large segment of the British populace who did not support action in holding the Suez Canal simply because holding it served a much lesser purpose by 1955.

This is going to sound like a cop out answer, but its their massive, and powerful navy that allowed British global presence. Britain was a naval power house under Queen Elizabeth, during which English colonies were founded in the North America. Under the Stuart dynasty, the English Navy began to fall behind, during which the Dutch took over the Naval supremacy. It was during this time the Dutch expanded greatly. However, this Dutch navy went to England, and to the British when William of Orange took the crown of England, Ireland and Scotland starting another set of global expansion by the English. The navy projects influence and military power over a far from the nation's territorial borders, allowing trade to grow and eventually project power economically(and eventually militarily in order to protect its economic interests).

The two World Wars were important factor in the decline of British Empire. The two wars drained the treasury and spread Nationalistic ideals in British colonies. Without a strong navy to project its power (Not to mention United States began to overshadow the British in terms of military), the British Empire was forced to concede independence to various nations, the most important being India. Without the money from these former colonies to support its naval supremacy, the British Empire declined to what it is today.