If you are flying AA you will be flying into Terminal 3. With a US passport you can use either the e-gates or the non-EU passport line in which the UK no longer even stamps your passport (you still give it to them and they scan it), it's a quick process assuming the line isn't too long. If you're in J/F the fast track line is almost always empty or near-empty. I fly to London regularly and it's now one of the least stressful border crossings.