Sightseeing during airport change.

You will not have time to do much more than switch airports, unfortunately. The speed with which you can get out of the airport will depend on number of arriving flights. I have had it been as little as 20 minutes, and as much as slightly over an hour. Then it's a good hour (depending on traffic, roadworks, breakdowns, etc.) from Heathrow to Gatwick. Then you have to check in, go through security, and get to your gate. Typically they say to allow two hours before your flight to get that stuff done. So that eats at least three of your maximum of five hours. I would go straight to Gatwick and plan to sightsee on another trip. The best and cheapest alternative from one airport to the other is probably the National Express bus (about 20GBP each). You can buy tickets online before your trip, but there is really no advantage to doing so.

And to add to Nancy's suggestion of the National Express coach to Gatwick, it is a long walk to the bus station at Heathrow so bear that in mind and don't dawdle.

I generally agree that your transfer is too tight to swing through London proper. The completely safe and prudent thing to do is to proceed directly to Gatwick. HOWEVER, if you are a bit nutty, AND willing to take a risk (note that many on this board are not), AND if you clear Heathrow lickety split, AND if you can quickly propel yourselves outside to grab a cab, well, I say go for it. Tell the cabbie you want to swing through London and glimpse Big Ben or the Tower or whatever it is you are dying to see on your way to Gatwick. He'll think you're crazy, but for the fare he'll do it. London cabbies are super knowledgable, really unlike anywhere else in the world, so you don't need a more expensive limo driver to be an impromptu guide. London traffic is hideous so you won't have time to get out and DO anything, but you'll get to ride on the other side of the road and catch a glimpse of the storybook city. My husband and I did this once when we were young and crazy. We literally hopped out of the cab and stared at the Louvre for 5 minutes just so we could set foot in Paris. We still talk about that awesome moment! And of course we have returned to Paris many times since. So if it suits your personality and you can get to Gatwick in time for the next flight (2 hrs ahead, ask the cabbie), live on the edge! Online check in and only carry ons can cut the arrival time to 90 min. ahead.

Thanks for the input. We will just play it by ear and decide when we get there if we want to try it. Hopefully Sunday morning won't be too busy traffic wise and we can get a cabby that wants to make it happen.

Hiring a cab to take you from Heathrow into London, drive you around, and then out to Gatwick will cost you an arm and a leg. What you could do is take the Heathrow Express or Heathrow Connect to Paddington (15 and 30 minutes into the staation from LHR respectively), then hire a cab there to show you around for 30 - 45 minutes or so on the way to Victoria Station. Hyde Park, Kensington Gardens, Buckingham Palace, and other sites are all within reach, as are Big Ben and the Parlaiment buildings for a quick drive-by. At Victoria you catch the Gatwick Express (30 minutes) (check the website to make sure they aren't dooing engineering works, which they often do on Sundays. June2, for example, the Gatwick Express will not run, but there are other trains that go to Gatwick.). Let's say you get through immigration in 30 minutes. Then you have 30 minutes (immigration line) + 30 (HR Express) + 30 (sightseeing) + 30 (GW Express), or 2 hours, leaving you 2 hours at Gaatwick and an hour of wiggle room (which is generous).