Flew on BA
734 to Venice from LGW
earlier this year. I'm a registered online user with BA
and as BAjax says you can check in twelve hours prior to departure, picking your seat from those available. Anything ahead of the overwing exits in EuroTraveller is good as BA
can change its cabins from Club to Traveller with adjustable seating, so pitch is good to take account of this. Further back its less as the seating is only ever used for Traveller.