Delhi-London/London-Delhi is a very do-able trip for an European/non-Indian. Done the research myself too sometime ago, and fully agree with hvkumar about Pakistan and Afghanistan being off-limits for us Indians. Travelling through Turkey is also a tough call; getting permits to drive are supposed to be a nightmare.
Also, exporting a car from India is expensive. A deposit of twice the new purchase value of the car has to be made with the government as surety that the car will be brought back into India again. For getting the money (or forgetting the money?), your guess is as good as mine about the "when".
It's easier to become a British/French national, buy a UK/FR reg car, and then drive it to India and back.
Other people are also planning similar trips all the time. Read
London to Delhi by Road in April 2010 ... - Thorn Tree Travel Forum - Lonely Planet and
Pakistan: Delhi to London through Pakistan, thorn tree, guerba.