Mmmmm... flights operated by FLYBE so why would there be a BA lounge in the South terminal?
The FLYBE lounge is fine for half an hour read of paper / cawfee / juice /etc.
The BA TKTS can be cheaper usually last-min and of course include baggage etc etc.... FLYBE charge for booking, seats, baggage, etc etc.
The main use of the BA flight number tho' provides a level of protection in the event of missing a BA connection, I would suggest..... Not sure if it allows checked-thro luggage?