Originally Posted by Globaliser
I think that the point is to stop people arriving on an international flight, pretending to do a direct airside international-international transit into the mixed departure area and then getting on a domestic flight without passing the immigration control. Don't know all the details of how it works at LGWN and EDI, though - not been to either airport enough.
That's exactly it. Most (but not all) nationalities do not need a visa to do an airside transfer in the UK. So, seeing as photo ID is not required for domestic flights, you could, in theory, meet someone in the departures lounge, take their BP, and hop on a domestic flight. Hey presto! You're in the UK, without a valid visa.