How successful would India be as a country if Britain had not colonized it?

Realistically, British occupation was probably good for India in the long run. While being conquered doesn't seem like a good thing, in this case it was the lesser of the available evils. Nations are not colonized if they are strong, and at that point in history India was not particularly strong. So, they would likely have been taken over by someone. And as far as such things go, over all Great Britain was relatively benign. They weren't saints, but they weren't complete monsters either. And after Britain withdrew, strong ties were maintained, giving India support and export markets they may not have had completely on their own.