We have a 7 hour layover in London which will involve transferring from Stansted (where we're arriving) to Gatwick (where we're going on to Rome). We understand to go via train we have to connect in London at Liverpool (?) and then we transfer to get to Gatwick. While we are in London, we're wondering if we would have time before taking the connection to Gatwick (a few hours?) to see something of the city, and if so, what do you recommend?
If your plane arrives on time and you don't have much luggage to haul around, you should plan on having no more than about 2.5 hours to see some things. Plan on 1.5 hours to get through passport control, etc & the ride on the Stansted Express to Liverpool St. Station. You'll then need to get to Victoria Station to catch the Gatwick Express (catch the train at least 3 hours before your flight time to Rome). The Circle Line (yellow) Underground (tube) goes from Liverpool to Victoria station. En route you could get off at the Westminster tube station & go up to have a look at the Houses of Parliament & Big Ben. Then it's just a short walk to Westminster Abbey. There's a taxi stand at the Abbey & you could grab one for the short ride to Victoria after your visit. If you're really pressed for time, take a taxi between stations and tell the driver to go by the above sights, plus maybe Buckingham Palace or whatever else you may want to see that's not too far out of route.