ePassport Gates LHR

What you did was not passport control. It was part of the security screening.

When you arrive in the UK, as a US passport holder you can use the egates. Same procedure.

When you leave, there is no exit immigration like in France. You will not have to do that with your passport if going back to the USA. You will scan your boarding pass just prior to security (no photo). That will get you into the security area. From there you go to your gate.

On departure from Heathrow you don't go through a passport check - it's check-in, a gate where you scan the barcode of your boarding pass, security check, and then check at the gate (where they will match passport to boarding pass).
And yes, there are multi-hour delays. We flew from T2 on Tuesday and checkin/bag drop took an hour 45 mins (flying economy) and security (passing through the boarding pass check barrier and the actual scan/search) was another 45 minutes. This compares to 20 minutes when we flew in January. I assume a mix of half-term holidays and lack of staff.

Note also that Heathrow has, per a very detailed Flyertalk article about the various transfer in Heathrow, "conformance" wherein you must hit this point where you are scanned with your face to passport no less than 30 minutes before your scheduled departure, else you may not be allowed to board. This comes into play in transfers. The base article is a few years old so I do not know if this time frame has been changed.