Why is it 5 : 30 p. m in india and 12 : 00 noon in london ?

Because after the sun is 'overhead' in India, it takes roughly 5-1/2 hours
for the Earth to turn far enough to place the sun 'overhead' in London.
Our clocks are built to match what the sun appears to do.

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.