I'm flying from Rome (FCO) to Heathrow (LHR) before heading back to The States. My layover at LHR is 5 hours, and I'm not dealing with checked baggage. I've used the Picadilly Express before and am very comfortable with the London tube system, so I'd like to leave the airport for a quick pop-up at the Parliament/ Westminster Abbey stop to walk around a bit. Am I crazy to think that 5 hours is enough time to do this and get back to the airport and checked in again?
Even though you're familiar with the London Underground, five hours simply isn't enough time to go into London. Even with no checked luggage you should be back at LHR three hours before a trans-Atlantic flight.
At best you'll have time for a 20-minute taxi ride to Windsor, a quick look at the castle, and a taxi ride back to LHR.
Heathrow is infamous for developing long lines both at immigration and then again at check-in and security. Windsor is really your only option and it would be tight. There's lots to do in the airport- plenty of shops and resturants.
I wouldn't risk it. We almost missed a flight at Heathrow even though we had a 12-hour layover and we were back in the terminals more than 3 hours in advance.
Yeah, I think that would be crazy. Schilpol? Yeah. Some other smaller airports? Yeah.
Heathrow is just too big, and way too concerned about security to even mess with it. (Not to mention delays, etcs other have mentioned.)