I just travel to London. Here is my experience, In Heathrow airport go down to terminal 2, at the ticket machine puchase the Travel ticket, it costs 6 pounds per person. You need to take the Picadilly line, this underground train stops 25 to 26 stop and will take about one hour to king Cross. No need to change train, it is very easy to take.

Hope this will help anyone going travel from airport to king cross.

I just travelled to London and here is my experience. At Heathrow Airport, in Terminal 2, go down to the Underground station and purchase a ticket at the machine; puchase the Travel ticket at a cost of 6 pounds per person. 

You need to take the Picadilly line. This underground train has 25 to 26 stops, and takes about one hour before arriving at King's Cross. There's no need to change trains and it is very easy to take.

I hope this will help anyone who's going to travel from the airport to King's Cross.