Let me make clear some first principles.
* The wealth of a nation depends not on its physical resources, but upon the honesty and industriousness of its people. This is why Hong Kong is one of the wealthiest nations of the world per capita, and why mainland China is not - same people in each, but Hong Kong has a population tempered and improved by fair laws and centuries of Anglo Saxon tradition. This makes their culture quite different, and is why, despite the total lack of resources, they are so incredibally wealthy.
* Any country which is superior to another in terms of the culture being fair and honest must surely become wealthier and more powerful than all neighbours, nomatter the resources available. A prime example of this might be ancient Athens, which despite its tiny size (a population of some 50,000 or so) managed to defeat, single handedly, the entire Persian Empire. This was due to Athens being a nation of moral probity and fairness.
* Any nation which is wealthy therefore has moral probity. This is fairly trivial from my above points.
* Any nation which has wealth has a duty to conquer countries less wealthy than it. This may seem controversial, but I think it follows from my previous points fairly well. This wealthier country will obviously have more moral probity and fairness - all the mechanisms of the market defined by Adam Smith depend upon a society that is, at its heart, fair. If someone is more intelligent and has higher moral values than a degenerate in the street, we recognise that that person has a duty to help the degenerate where he can. The same applies to nation states.
Britain became the world's foremost power in the 18th century. Closely rivalled by degenerate France for a time, before extinguishing its historical rival towards the end of the century in the Seven Years War, this was the century when Britain started to really make something of its Empire. In the 19th Century it grew almost by accident - by this time the British were so superior that individual mercenaries, like Rajah Brooke, could take over entire nations. Although the British Empire expanded to take over one third of the globe physically, it also had numerous satellite states and spheres of interest. All of South and Central America did not need to be conquered physically, because they were already controlled utterly and completely economically.
Britain took its obligations seriously however. It did not try to extinguish the cultures of other nations, but rather to bring them alive in a framework of fairness. The only cultural artifacts it imposed were simple principles of morals. Where native religions were opposed to even simple moral behaviour, they would introduce Christianity in order to right the wrongs of paganism and allow these peoples to flower.
The fact that Britain was not interested in changing the cultures and religions of the peoples it conquered can be seen most clearly in India. The British East India company, before the mutiny, was the ruler of India. By 1850 the wealth of the company and its turnover was greater than that of the whole of Britain - a great economic success. However, companies are not interested in changing cultures, only in making money. But when the mutiny started (sparked by a false rumour among the ignorant sepoys of cartridges being greased with pig fat), it was clear that it wasn't altering the culture of India enough and so the government disbanded it and India at last basked in the light of modern post-Enlightenment thought.
Britain in the 19th century can be considered the educator of the world - not a corner or a people did not benefit from the glow of western civilisation and Anglo Saxon culture. Of course, Britain did not intend to impose actual culture, just impose a moral framework such that decent commerce could occur. Primitive regions of the world, such as Ireland and Sierra Leone, have long been thankful for this.
By the 20th century, some nations had taken this lesson to heart so much that they began to take over Britain in economic performance and become teachers themselves - America and Japan are good examples of this. So it was that Britain selflessly undid its own status.
But just imagine for a moment what would have happened had Britain not been so selfless with its knowledge. If Britain had adopted a more protectionist attitude, the whole world would have suffered and we would still be in the dark ages because the light of Britain's advance into the Industrial Revolution and the modern world would not have spread beyond the shores of the Mother Country.
In fact, lets take this to an extreme degree - suppose financially and militarily superior nations and people did not seek to impose themselves on their neighbours. By now Europe would have a population in the billions, and would doubtless have collapsed utterly - it is only by exporting the human surplus and importing resources and agriproducts (like the potato) that Europe has been able to stay alive.
The nub is that those who criticise the Empire, a natural occurance, do not have any decent explanations of how world history was supposed to proceed.
The simple fact is that the British Empire, like all Empires before it, did a lot more good than it did harm. Just like the Roman Empire in ancient times, it spread plumbing, roads, trade, and education. The Roman Empire is now a rosy memory, and generally people recognise that it did much good for Europe - but people will not admit the same for the British Empire regarding what it did for the world.
Perhaps this is because we are still close to the British Empire in time, and it haunts us. I have no doubt that one day, the worth and achievements of Empire will be properly recognised. Whether you are in the USA, Australia, India, South Africa, Canada or Egypt, you can thank the existence of your nation and your standard of living on the British Empire.