It's possible to travel from Europe to India overland by train and bus via Turkey, Iran & Pakistan. It will take a minimum of 2-3 weeks, and you should consider it as an adventure or expedition than a routine way to travel there. Administratively, the main issue is getting an tourist visa for Iran, although this is becoming easier - see the London to Iran page for agencies to contact to get one. The logistical problem is building an itinerary around the weekly train from Istanbul to Tehran and the twice-monthly trains towards to Pakistan border. Finally, there are security problems in southeast Iran to be aware of - see the official travel advice for Iran and Pakistan at the British Foreign Office website, www.fco.gov.uk.
http://www.seat61.com/India.htm#London to India overland