The combined UK economy could always upgrade the Calcutta minor, like a tax on 1st turn purchases to prevent radical changes to first turn moves. The important thing is the UK Pacific economy wouldn’t collapse if Calcutta fell.
The Norway IC supplies its own troops to fight off nasty incursions from east and west and
bringing pressure on northern USSR while the airbase brings extra scramble ability to SZ112 fleet and the ever hopeful Sealion - admittedly not turn 3-4 but maybe later. Still thinking it through.
You will have no choice but to reinforce it sometime by sea or air because once all three allies come down on it and they will, I promise you ten infantry per turn is nothing.