Originally Posted by Ldnn1

The basic answer is simply that BA can't fit everything into T5, so some routes have to go from T3. LYS is one of them, along with some others including MRS, LIS, PRG, VIE, WAW and BCN. The reason why it's those and not others isn't absolutely clear, but it's been suggested they are either routes with fewer BA-BA connections, more O&D and/or more oneworld connections than other routes. Some are/were also a legacy of former bmi routes previously in T1 (think LYS might be one of those but can't remember).There was a re-jig last October when a sub-group of longhaul also moved over to T3, with some shorthaul moving back to T5.



Nothing to do with gate assignments here. You can be absolutely sure which terminal your flight will depart from. Occasionally there's a re-jig as mentioned above, but those moves are announced months in advance.