I'm going to be arriving in LHR on AA, and then flying to Amsterdam on a separate ticket. I will be flying out of either Terminal 4 or Terminal 5, depending on what airline I use. I believe I will have to exit customs/immigration in LHR, and then need to re-enter through security...? On the LHR website, I find information that says buses transit between terminals every 10 minutes but nothing telling me how long it takes to get between terminals. Practically speaking, on a weekday at 9:00 AM, how long does it take to get between T3 and T4 or T5? Also, am I correct about get off the plane, clear immigration/customs, re-enter security?