I presume that excludes T5? I can't see how someone can get past conformance in T5. In the case of T1, T3 and T4, it has long been the case that there is free movement between terminals, though I hadn't noticed the "let's go shopping signs". (The exception being between T1 Domestic and International). So it's perfectly possible for someone arriving in T5 to pop over to T3 for a bit then go back to T5. Or even for someone arriving in T4 to exit the UK Border in T5 to avoid a HEX train change. But that would not allow a T3 passenger to go to T5 temporarily unless they had a flight from there.