That has always been my understanding however after my trip home from JNB last week. Standing near the GA I heard a phone call where the GA asked someone to "hold off on upgrading anyone" (from other conversations got the sense there may have been weight/balance considerations, i.e. agents unable to seat couples together despite a light load in Y) -- 10-15 minutes later and about 5 minutes before boarding I got the "you've been upgraded!" notification so I politely approached GA #1 to ask for a new BP... that GA passed me off to GA #2 with a "he claims he was upgraded" -- GA #2 printed a new BP but also seemed unaware that any UGs had been processed.

Based on that I'm wondering if at least for small INTL outstations UA is having someone in Ops/SOC run upgrades rather than doing it at the gate... but LHR is big enough I also wouldn't lump them into the "small outstation" category.