What is the most affordable and efficent transportion between Heathrow and Gatwick?
Colleen; don't know about affordable/efficient but National Express (coach/bus) operates a shuttle between Heathrow/Gatwick and is your best bet. Should cost about $35 and you should allow 3 hours from arrival Heathrow to departure Gatwick. Check National Express website for exact details/scheds/$.
First of all, three hours is not enough time to make the transfer--unless everything goes absolutely perfectly (on time arrival, quick passage through immigration/passport control, no traffic delays). IMO, four hours is the minimum. And longer would be better.
Taking a direct National Express coach is the most efficient, but also the most expensive (£19).
If you don't mind schlepping your luggage and have enough time, there are cheaper options. (1) Take the Tube to Victoria (£4) in a little under an hour. Take the Gatwick Express train from there (£16.90) in 30 minutes. Or take the Tube to Victoria and a Southern train from there (£8.90) in 40 minutes. Cheapest of all would be to take the Tube to Victoria and an easyBus from there. Booked in advance at www.easybus.co.uk, the fare can be as low as £2. However, it will be a slow ride, especially if traffic is bad.