Originally Posted by corporate-wage-slave

Yes, this is a bit of a historical relic, and actually is not to do with the bus transfer itself. In the HAL operating procedures agreed with the HMRC, there was a requirement that anyone passing from domestic arriving aircraft could only pass into international flight connections if they were "bona fides". This was connected with the previous set up of duty free purchasing. Now this is a nonsense at T5 and only made limited sense at T1. Still there is a HAL checkpoint by A8 (or thereabouts) where HAL agents check that you really should be in that part of the building. What's more HAL seems to put some of the least well trained staff at this point - they probably would know AA is T3, but clearly QR is a stretch too far. Sometimes you get an agent who does know LHR reasonable well and a quick glance of your paperwork is ok. If you approach them brandishing the boarding pass - so they don't need to ask for it - it can help if it is clear you know what you are doing. What's really bad about it is that the one thing which would be useful would be to stop T5 connecting passengers going through that point, but they often don't think that through either. You don't specifically need a boarding pass, you need bona fides of some sort. So an email on a phone is OK.