A number of things to bear in mind about the British Empire.
The British Empire was not just run by the English. The Scots, Welsh and Irish also played a role in creating and maintaining the British Empire. Plenty of Irish fought for the British Empire. For an abvious example look no further than the Duke of Wellington, who was born in Dublin. Granted the Irish were victims of the Empire but they were also agents in creating the Empire. By the by, the Scots tried to start their own Empire in the early 18th century by invading the Americas. They failed terribly, bankrupting the country, which caused them to sign the Act of Union in 1707. Oh and Scotland is named after the Irish tribe, the Scotii. So you could argue Scotland is an Irish occupied territory.
The main cause for the collapse of the British Empire was not a sudden growth in nationalism and resultant wars of independence, but was in fact World War 2, when Britain throwed the Empire against Nazism. Economically and politically Britain never recovered from World War 2, leading to the collapse of the Empire.
The Growth and Development of the British Empire should be placed in the context of the time. The notion of the nation state is a modern development, developed in the nineteenth century. Prior to the nation state, the empire was the prevalent and standard form of government. The Brits are not unique in having an Empire, they are only unique in having the largest and most 'succesful' empire.
If it wasn't for the British Empire there would be no USA, Australia, New Zealand or Canada and everyone in Europe would probably be speaking German.