It might be curious to see the ratio between the various allied Indian states and India proper. In OTL 1976, there were just over 613 million people. 8 million Indians, thereabout, died during the independence war. Population growth would have been near 0 for the entire length of the war as well. We might be looking at a population of around 500 million at the worst to 550 million at best.
Then there is the matter of population density. With most of people populated in OTL in Bengal and along the Himalayan rivers or the southern peninsula's western half, I'd lean heavily toward saying that majority of the population is still in the North. (which was more battle-scarred than the south) But excluding Bengal and Assam, along with the southern states, Kashmir, and Baluchistan, the ratio might dip as low as 2:1 in favor of India.
Then the question comes to the conquered territories and how many of them end up rebelling or supporting the Indian regime. I think an optimistic number might be half supporting and half against, but I wouldn't be surprised if only 10 percent support the Indians because of their aggressive methods.
I'm not sure how enthusiastic the rest of India is for this war as well. The enthusiasm will be, in the end, the key. Indian generals are already worried what will happen if they give the Commonwealth too much time to reinforce. Even then, a 4:1 or a 5:1 advantage can be overcome, and a 2:1 or 3:1 is nearly an even fight considering that Britain has better technology and a larger navy to be brought to bear. The farther south they are pushed, the more the Indian army comes into range.
http://www.populstat.info/Asia/indiac.htm
http://www.wrsc.org/sites/default/files/images/2011/india_population_densities_est._1995.gif
Links I used for both.
On a side note, what would the naval disposition be of the various major combatants? (India/Japan and Great Britain/Canada/South Africa/Australia/New Zealand/Portugal et. al) There hasn't been any naval combat to prove the viability of carriers over battleships. There definitely would be a transition, but carriers might be treated more akin to floating ironclad batters of the Crimean War, slow but carrying large amount of aircraft, instead of a faster vessel to spearhead attacks. Would that come into play, and would we see latter-day battleships (successors to OTL Yamato/Lion/Montana) being built, integrating guns and armor with the missiles of the period? Just a thought, and I'm probably completely wrong.