We will be traveling through London on the way to Italy. We need to change airports both directions. We will not have much luggage, online carry-on. What is the best transportation between the airports? Both directions have about 5 hours between arrival at one airport and departure from the other. Is there any sightseeing of London that can be done with that amount of time? Hiring a driver/tour guide to see one site would be ok if the price isn't too astronomical! We have never been to London.
Unfortunately, you aren't going to have time to see London. Your primary options are to take the National Express coach between the two airports ( see this link) or to arrange private transfer with a car service which will be expensive. You have to get off the plane and go through immigration on arrival and that could easily take an hour. The trip by coach takes about 90 minutes. When you arrive at the airport, you need to go through security and the length of the queue will vary depending on the time of day. And you need to be at your gate in time to depart.
As Laura says, you really cannot do this. In fact even though the two airports look so close on the map, you really need to head for Gatwick as soon as you are out of the Heathrow formalities. How expensive pre- booking a private transfer is compared to using the scheduled bus transfer will depend on how many there are of you?
You can see more options here
You have enough time to make your Gatwick flight, if there are no delays getting into Heathrow. Getting through customs, getting to the next airport, and being there on time for check in doesn't leave you time to visit anything. The National Express Coach is very good, if the schedule works for you. If you fly into Terminal 5 at Heathrow, you can buy your ticket using the machines, my American credit card worked just fine. There is a desk, but there's generally a queue, and using the machines helps save time. You can save money by buying your ticket in advance online, but you have to read the restrictions in case your plane is late and you miss that coach. Good luck!
I am curious how the bus time compares to Heathrow Express to Paddington, taxi to Victoria station, and Gatwick Express fom there. Probably more expensive, but would it be faster, if they arrive at Zt5?
Sasha, theoretically if you had a car waiting for you outside your Heathrow terminal it could get you to Gatwick in rather less than an hour. However in practice the bus having to pick up and the expected delays on the Heathrow to Gatwick route means 90 minutes is a better estimate.
It is just possible that if you add the travel time for the via London route it looks a bit better, but in practice when you add the connection times such as waiting for the next train, because you just missed the one you wanted (plus the unknown time for Paddington to Victoria), then it is very, very unlikely to be quicker.