Hi - Newbie here so help appreciated

I've booked a holiday vai Barrhead Travel to St Lucia , flights with BA (economy - Which i have added to my booking in my BA Exec account) on a 777 , two questions / queries really

1. When i go to try and select seats on my BA - its advises that seat availability is only available at Check-in - is this correct and if so whats the likliehood that i'll be able to get two seats together - am guessing its because the agent has booked cheapest option in Economy ??

2. I know it's economy but any suggestions for best seats / area ??

Thanks in advance
Iain