You can choose your seat (but not check in) via 'Manage My Booking' and then 'Choose your seat'. However, as others have said, it ain't cheap!! We are going on holiday to the USA later this month and the cheapest seat is £77...each...each way! The most expensive is £99!!!! I just wait until the moment the 24hr 'check in' slot starts and choose my seat then for free. You can normally get something that suits you.
No I haven't, Tilly. I've only flown BA once (to the Turks & Caicos Islands, 2003) and can honestly say they were the best airline I've ever flown, with Air Canada a close second.
I don't know why BA get such bad press. Sorry for going off topic.