Heathrow’s Terminal Three is used for American/British Airways flights. My biggest frustration here is that they announce your gate about 15 minutes before boarding. There’s almost no seating, so you are left to stew in mediocre to poor restaurants and duty free shops until they finally post your gate. At the point, there is a mad stampede to a gate that is most likely at the other end of the terminal. Be prepared to show your passport and boarding pass at least five times before getting on the plane. One redeeming factor: Terminal Three has a very nice Admirals Club that can save you from the aimless wandering.
A couple of years ago, I had an eight-hour layover at Heathrow on my way to Istanbul. I stayed at the Yotel there, a tiny hotel inside Terminal 4. For around 100 dollars, I had a small room, perhaps 10-foot square, equipped with a bed, TV, and bathroom. It’s a pleasant place for a few hours of sleep after a transatlantic flight, but the next morning, I needed to transfer terminals before the train was operating, and so had to take a city bus to the other terminal. It took 45 minutes to transfer to a terminal that should have taken less than five minutes.