Is the sterile area only a UK thing then? I had never experienced anything other than it until I flew into CUN on Aug 23rd (BHX-CUN-BHX on TOM568/9) and there is definitely no sterile area at CUN.
In fact when I arrived I was really confused thinking that they had accidentally let everyone into departures but no, this is how it is done there. You arrive through the jetbridge, walk to the top and then cut across the departure lounge to go through to the 'Migracion' Immigration area.
We had friends on the inbound LGW first choice flight on Tuesday, we were flying back to BHX and we managed to get a chat with them, in the departure area before the went off to immigration, it was so weird.
Edit - actually come to think of it I am wrong! I have seen this before at EDI, I was staying with my brother for a week and my mother was coming for the following week, she arrived on the inbound BE EMB195 and we managed to get a chat with her then