That sign makes absolutely no sense and has clearly been written by somebody with no understanding of the cabin classes/airlines/statuses involved. It's also completely not on brand, so I'm assuming shouldn't be visible for passengers.
Basically what the sign is saying is that Pre-Flight Dining in the T3 Galleries First lounge is open between 18:30-21:30 for BA Premier passengers, BA First passengers (plus one guest), BA First GGL passengers (plus two guests), Qantas First Class passengers (plus one guest), Qantas Chairman's Lounge passengers travelling on Qantas (plus two guests).
The guests for 'normal' First passengers goes against the general policy, and the fact that CCR Cardholders aren't mentioned is poor (as really this should be branded as a Concorde Dining Room, and open throughout BA First departure times).