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.
When it is 12:00 noon in London, it is 5:30 p.m. in India due to the difference in time zones. This difference arises from India's location at UTC+5:30 while London is at UTC+0. Therefore, India is 5 hours and 30 minutes ahead of London.
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