Flying into Gatwick, and then planning on flying out of Stansted on Easy Jet to Ljubljana. Arrive 8:40 am, and flight out of Stansted leaves at 13:40. Is that enough time to make the transfer? What is the best way to get to Stansted from Gatwick?
Check the National Express website, www.nationalexpress.com. It will show you the bus times between the two. That should be plenty of time to get from one to the other, though. Stansted is a very easy airport to figure out once you're there and it won't take much time to get checked in and through security.