I'm flying from Seattle to Warsaw with a connection in London (Heathrow). We arrive at Terminal 5 and depart at Terminal 3. With a 90 minute layover, I'm concerned about getting from terminal to terminal, as well as the time spent with Customs. Has anybody done this? Any tips?
Yes, we have done this. You simply follow the purple "Flight Connections" sign directing you toward T3. They will lead to to a bus for the transfer from T5 to T3. You will stay "airside" and you do not go through immigration or customs.
I timed it from the time we exited the plane, and it took us 35 minutes from gate to gate.
This is assuming you have carry-on luggage only, or baggage that is checked through to Warsaw.