By 1940, India was a drain on Britain

Economics was the primary reason Britain left India. Gandhi, you said?
Losing Hope, Glory and Assets - WSJ.com

Still, it is not wrong for Mr. Clarke to assign the blame for imperial collapse to wartime commitments. And money had a great deal to do with it. The war, Mr. Clarke notes, "left India a creditor on a vast scale, with Britain owing it huge sums in the form of the sterling balances." This fact meant that London actually owed New Delhi some £1.3 billion pounds (or $5.2 billion in 1945 dollars). The empire had conferred many benefits on Britain, but by the 1940s its administration and defense were a net drain on London.