London Heathrow

It depends a bit where you are going beyond Heathrow, but off hand, I would say "No". You would need to clear immigration, then get into London, so maybe 90 min. to two hours, then be back to Heathrow in plenty of time to get through Security, leaving about enough time to take a picture of Big Ben and get back on the tube.

If I remember correctly from Heathrow to Piccadilly Circus on the tube is more than 45 min. each way. Let's say two hours round trip to be safe. You will need probably at least 30-45 min. to deplane and get to the train from the moment you land. The other question is how long before your next flight you need to be there. Are you flying within Europe or coming back to the US? You can't show up back at the airport 10 min. prior to departure. You need to be at the gate 45 min. prior. So to summarize: 45 min. to deplane and get to the train. 2 hr round trip travel on the tube
75 min prior to departure back at the airport. You might have about one hour to visit the City. Have a good time.

No, it really isn't. As the previous posters noted, it's a good hour into the city and another one back,and you will be hauling your bags with you (except for any checked bags that will probably be checked through to your final destination). You never know how fast the security line will be moving when you get back, and you have to be at the gate in an adequate amount of time (check your airline for their rules). On my flight last month, boarding started at 9:45 for an 11:00 flight. There are plenty of stores to poke around in at Heathrow, and I think you will be surprised at how quickly the time goes.

Totally agree, I would be a wreck worrying about getting on my flight and would not even enjoy London, just my opinion.

Five hours in an airport waiting for connecting flight=long and boring. If you must go somewhere, catch a taxi and go to Windsor for a look around. You might even have your taxi drive around for a while, then return you to the airport. A tour of the village of Datchet (very close to the castle) then on to the castle area, followed by a drive around Eton would give you a good idea of that area. Don't risk problems by cutting short your arrival time before your next flight.

Thank you for the responses.
Dick