The unifying force of British Colonialism allowed the Indian nation to see itself as one. This was suggested by the previous post and is highly valid. In amplifying this, I think that one could make the argument that over time, the resentment of British rule helped to bring the nation together. The notion of being taxed unfairly, exploited both politically and economically, and the premise of "the outsiders" making homes within helped to drive the engine of Indian nationalism. Movements like the Sepoy Mutiny helped to bring light to the overall unfairness of British rule and bring major questioning to the Raj and India's role within it. The cultural divide between both began to emerge as nationalism began to emerge with imperialistic rule. Add to this growing divide, the events of the 20th century around the world that were marked by the continual calls for freedom and democratic rule, nationalism was almost inevitable as well as the end of British imperialism.
Before the British came to India, there was no sense of India being one country. It was, for the most part, split up into a number of different political entities.
Gradually, however, being under British rule moved the Indian people towards a feeling of nationhood. This is largely because Indians from various areas were able to feel more kinship to each other when they had the British as a common enemy.
Some scholars also say that Indians moved toward nationalism in emulation of the British (because the British themselves were nationalists so the Indians learned it from them).
To claim that British united India in one country is similar to saying that the British united the 13 separate countries into one united country that fought the American War of Independence.
India had existed as separate country for nearly two thousands of year, united by its geography and its culture. Politically also very large part of India have existed as single empire long before colonization by British. If we want to get too technical, British never ruled over whole of India. There were around 600 independent states within India which were independent kingdoms with which British had entered in to treaties. Also there were territories within India that were under control of countries like France an Portugal.
Of course the spirit of nationalism among Indians which inspired them to free themselves of British rule was very much the result of oppressive rule and insulting behavior of the British.
Also I do not understand in what way British people were more nationalistic than people of other countries. British people had their own share of division and conflict based on religion (Protestants and Catholics), region, economic classes and struggle for power among members of royal families. Most certainly, Indian nationalism did not emulate any part of this kind of nationalism.
No doubt British industry and business exploited Indians with the support of British government. But they also exploited the poor people and industrial workers in UK. It is because of this that British textile labour cheered Gandhi enthusiastically when he visited them during his U.K. visit.
Coming to the British view in 1800's of Indian culture, it is very clear that Britishers were highly prejudiced against Indian culture and people. This was was largely due to arrogance which develops quite naturally in oppressor towards the oppressed. However it is unfortunate that many of the misconceptions and falsehood spread in the past by Britishers about Indian culture and achievements still continues mislead many people in western countries.