mtkeller

star_wonder is correct about not clearing immigration, since the "passport check" you describe is conducted by the airlines or security contractors and not by the UKBA. It is a cursory "Yeah, you've got a passport that's valid, so we won't get stuck sending you back after being denied entry to the US for lack of a valid passport." check, not an actual passport control with entry stamp conducted by the UKBA. These passport check things generally don't add any measurable time. However, UKBA can easily add an hour if you arrive at the wrong time. (And I've done that twice in the month of May so far).