The chap at GLA checking boarding passes told me it was "for customs reasons" but I suspect, with others, it's partly just habit with the piers being completely self-contained - a leftover from the old days with separate domstic & intl security?
He also told me "they" (Bmi? GLA/BAA?) are working to change the set-up so intl pax can use the proper Bmi lounge.
I thought I had read, by the way, that the UK would be re-introducing outbound passport control in November. Obviously it hasn't happened yet but is this pleasure one we can look forward to?