It really depends on the fare booked. If you have a look in manage my booking at your long haul flights and if it shows P then it is Business Lite which means no lounge access and no Al Safwa. Any other code J, C, D, I or R is fine for lounges. Any lounge access on a P fare will be with your BA Gold card and that translates to the Qatar Platinum South lounge or the Platinum and Gold lounge North.
If it is a P booking, it usually lets you pay upto Al Mourjan in advance but I don’t believe you can pay into Al Safwa on a lite fare.