It's a US regulation, so if flying back from the US, it may be an issue regardless of BA policy. I found out when my wife/ daughter were in domestic F (redemption) but I was in exit row Y (paid), and although different PNRs, we boarded together and the GA said "no can do". The GA was incredibly rude about it, implying I was trying to trick the system, but I genuinely had no idea...

tb