I tend to agree that an Indian IC by the UK doesn’t delay japan for long enough, and in fact speeds up japan quickly after they inevitably take it.
I’ll give you a scenario that I’ve faced in past games:
UK1: UK retreats 1 inf. from persia to india using the transport, leaving 3 inf., 2 fighters (assuming 1 russian fighter is moved into it)
J1: Japan attacks china, killing the US fighter and infantry, tranports all available infantry to kwangtung, lands fighters there, and builds a factory in it.
UK2: Builds 3 inf. leaving 6 inf, 2 fighters
J2: Builds 3 armor, leaving about 7 inf, 3 arm, 3 fighters.
UK3: Builds 3 inf, leaving 9 inf, 2 fighters
J3: Builds 3 armor, moves inf. into French into china, leaving 6 arm, 3 fighters in Kwangtung
UK4: Builds 3 inf, leaving 12 inf 2 fighters
J4: Japan attacks Kwangtung with 8 inf (7 from kwang, 1 from french-indo china) 6 arm, 3 fighters. Japan takes india with about, say, 3 armor.
In my opinion, this delay of japan is not significant enough to warrant an Indian IC. And keep in mind, the 3 tanks/turn is only about half of the Japanese income. During this time it is likely ferrying in troops from Japan,
and making deep advances into asia and russia, possibly building a factory in manchuria to speed it up. Japan has really been delayed only slightly, as
only two turns worth of armor have been diverted to capturing india.
UK could also opt to build tanks every turn, however, they would be easily counter attacked by the japanese forces in kwangtung, weaking the defense of india and forcing UK to spend more money every turn.