The £350 offer suggests they've miscalculated the distance of your journey. This is the rate for flights of between 1500 km and 3500 km. If it's more than that the rate is £520 per person. I don't know where you were flying to, but check the distance and if it's more than 3500 km message them back and point this out. Unless you fly regularly with BA, insist on cash, don't take vouchers.