We are flying in to LHR at morning 7am and departing on 9pm flight to India.

We would spend 2 nights on plane with 14 hour between 2 flights.

Is it possible to have shower at LHR Terminal 3? We are flying economy class.

With 14 hours layover and flying in coach I suggest getting a hotel room for a day. With such long flights you are going to be greatful for an opportunity to take long relaxing showers, change your clothes and rest.

I'm with z, get a hotel room. I know there is a Marriot a short cab ride away from LHR. However, there isn't place to wander much outside the hotel, if that is important to you.

Alternatively, and very quickly, you could go into central London by taking the Heathrow Express which goes into Paddington station. I'd suggest getting a hotel within walking distance of Paddington for convenience. And a walk about the city would do you good after a long flight in coach.

To answer your question, I doubt if the airline would let you use their facilties. Airlines in general are strict, and rightfully so, about enforcing who gets what goodies.

Last summer I flew from Miami to Istanbul on British Airways and had time to take a shower at the Heathrow Airport. They took credit cards. Very private and they supplied the towel. I think it was Terimal 3, but obviously you will have time to find it.

A quick scan of the official Heathrow site (http://www.baa.co.uk/main/airports/heathrow/) shows:

Showers are available in the following locations throughout the airport:
Terminal 1
Before security

Terminal 3
Departure lounge - these facilities are currently unavailable due to development works.

Terminal 4
Available in the toilet areas on the Arrivals level, before security and further facilities are available after security control

Additional shower facilities are available in the Flight Connections Centre.