In London, the Greenwich Mean Time (GMT) is followed. The Indian Standard Meridian is located $$82\,1/2^\circ$$ east of the Prime Meridian. Thus the Indian Standard Time (IST) is 5 hours and 30 minutes ahead of GMT. That is why, when it is 12 noon in London, it is 5.30 P.M. in India. As we know the earth rotates form west to east the sun rays first fall on the eastern most longitudes. so, the countries which are on the eastern side of Prime meridian are ahead of it whereas western parts of Prime Meridian are behind it.