Originally Posted by
gpb_croppers63
That's not technically true as the UK does not have outbound passport control. In BRS, for example, domestic and international flights go from the same gates with no requirement for the UK Border Agency to check passports before boarding. As such, I can see no reason why an outbound international flight couldn't use a dmoestic pier in GLA. Arrivals are completely different as the UK Border Agency do check passports there but for departures, there is no need whatsoever.
It's a question of transit. As there's no requirement to show ID on domestic flights, you need a way of ensuring that the person who checked in (or cleared security) is the person at the gate. At mixed-departure airports, they take a digital photo, which is associated with the boarding pass on the computer system. This prevents somebody from entering the mixed departures area as an international-to-international transit passenger, then swapping boarding passes with a domestic passenger and ending up entering the country undetected.
However, the departure areas at GLA are (IIRC) completely contained within very separate piers. There's probably little motivation to spend money combining the two, given the relatively small number of international-to-domestic transfers at the airport.