So any time GB wants to sell tickets to passengers as 'GB', and not 'BA', I think you will find many people would be very happy to see customers make their choices accordingly!
But the customers wouldn't be able to make a 'choice' as BA (Mainline) will not (cannot?) fly to the same destinations as GB.
Unless BA (mainline) get their costs down low enough to be able to compete on the type of routes GB operates, the only way to have a BA presence on those routes is to sub it out to an outfit who can make them pay.