For shorthaul: select seats via
www.rj.com in advance (will only let you pre-select seats behind the wing at this stage), then make sure you OLCI as early as possible and as a BA GCH 99% of the time you will have access to rows 2-4 which will usually remain empty until the last possible moment. I can't remember the last time I didn't have the whole of row 2 or 3 to myself on AB shorthaul.