Originally Posted by
layz
What happens at LHR if you go through security and then discover you've forgotten your passport, or if you're flying domestic (and therefore decide not to bring a passport) and you suddenly find yourself needing to go back landside?
You can get back landside without a passport (as you noted if you are on a domestic flight you wouldn't have needed one) but I believe you will get some questions asked by an immigration officer and have to persuade them you are a UK national.
At least if you were supposed to be on a domestic flight you have the biometric data to reconcile you with so that would help I guess.