My great grand father served in France in ww1. He rose to the rank of Major Subedar, which was the highest rank an indian could achieve.
He was retired by ww2 but was called back into service. He was stationed on the Burma front during that time.
Apologies if this is the wrong sub for this, any direction is appericiated.
You could try The Churchill Archive Centre at Cambridge.
Top tier universities usually have extensive records and archives that they sometimes let members of the public use.
If not, you may want to try the Imperial War Museums in London, or the British Empire Museum.