Virgin Atlantic Airways Ltd.’s deal for 12 A350-1000 jetliners, announced Monday at the Farnborough Air Show, will come as a bittersweet victory for the model’s manufacturer Airbus Group SE. On the one hand, Airbus successfully staved off the challenge of Boeing Co.’s 777 to secure the $4.4 billion contract. On the other, a declaration that the A350 will be Virgin’s “future flagship” makes it less likely that the U.K. carrier will ever take six much bigger A380 superjumbos ordered 15 years ago. Virgin is not only purchasing the 366-seat A350-1000s for delivery between 2019 and 2021, it could also be in the running for a 400-berth version, known as the A350-2000, should Airbus choose to build the stretched model, Richard Branson, the airline’s billionaire founder, said at the Farnborough expo. “We’re always pushing for larger aircraft,” Branson said when asked about the bigger A350. “Size is good in the airline market as well as elsewhere.” Role in Doubt That rule seems not to apply to the 550-seat A380. Virgin Atlantic has repeatedly delayed handover of the jet, and Branson said even a decade and a half after the order was placed, it must “still be determined” what role a double-decker would play. He added that though the six aircraft remain in Airbus’s backlog, there is no obligation to take them. The composite, twin-turbine A350 is less costly to run than the all-metal, four engine A380, and will also be less of a challenge to fill as Virgin replaces its larger Boeing 747 jumbos. The new planes will initially be deployed at London Heathrow airport to bolster trans-Atlantic capacity, before taking on long-haul leisure duties at the U.K. capital’s Gatwick hub. Delivery of the A380s has seemed less likely since Delta Air Lines Inc. bought a 49 percent stake in Virgin previously held by Singapore Airlines Ltd., a major superjumbo customer that was the first airline in the world to fly the plane. The Delta deal has meant a return to the carrier’s North Atlantic roots and puts an emphasis on a high-frequency timetable less suited to the double-decker. Virgin Atlantic also faces competition on London-U.S. services from Norwegian Air Shuttle ASA, which uses the smaller Boeing 787 Dreamliner. The Virgin contract still marks a success for Airbus, having been won in the face of competition from both the 777-300ER and the 787-10.