You just highlighted the flaws when you make your statement, such as
"about India being a nation only after the British made it one, about it being a geographical expression, about the nature of Indian civilisation, think about what it involves."
Lets take the first one. Its reasonable to say that India as a single political entity after British unification of India. Prior to the British, the one empire to claim to rule over India were the Mughals. And they were an invader as well. Though there were small and large kingdoms, small and large empires, that existed through out most of history of South Asia, none of them ever claimed to rule over the kingdom called India, defined a "traditional" boundary and proclaim that all others within that boundary as traitors. South Asia prior to the British existed as many states fighting against each other and against invaders from North West, as all Indian invader came from the North West.
About India being a geographical expression, its no longer so. But prior to the British control, the term "India" is similar to Arabia, Persia and Meso America. These are not kingdoms, but geographical expressions. When I use the term Persia, we are talking about the whole Persian people, which include the Pushtans, Ajabajianis and Iranian, not just the people of Iran today. So in this sense, we can say that India prior to British control was a geographical expression. But its a name of a country today.
About Indian civilization, there was the Indus valley civilization, the Indian civilization after Aryan invasion and the comingling of Muslims invader and their rule over India. So India civilization today include the creation of Taj Mahal, which was the creation of a muslim Mughal emperor. But the term civilization does not equate to nation. I would say that there have always been a distinct Indian civilization throughout its history. But even if so, it does not equals to a historical Indian kingdom through out history.