Your status on AAnother AAirline will give you access to the lounge. But you should note that it is GB Airways (not BG) who you'll be flying with to CFU.

E class on BA is generally used for rebated staff/travel agent tickets in World Traveller (Economy). It could be that AA have used one of their codes as your flight was booked through them.

GB flights offer both a Club (Business) cabin in a 2-3 config, and regular economy in a 3-3 config. It depends on how full the Club cabin is as to where the start row is for Economy seating. There are some occasions where you can purchase an upgrade on GB for 59GBP but these opportunities are far less on GB Airways than on regular BA flights.