I know that the British Empire as a whole had a population of about 400 million at this point (as per 1918's Nations at War, and that total likely didn't include South Africa, Australia, NZ, Canada, etc. But I don't have direct numbers on India/Pakistan or on the rest of the UK's holdings with which to reverse-engineer India from. You can probably safely assume over 200m, though.
I've only got 1881 and 1931 unfortunately. Population growth accelerated slightly after WWI, so the 1914 figure is probably a little under halfway between them.