Originally Posted by
joshnesbit
how would it hypothetically if doing it in reverse, Edinburgh to Heathrow then to Washington? would you still do immigration at LHR?
No. There is no exit immigration. There is also no need to re-clear security when you get to Heathrow. Simply come off the plane, follow the purple signs to connections (if you are bussed from your Edinburgh flight look at the signs and don't follow the crowd into reclaim), scan your boarding card and head up the escalator into the departure lounge.