I'm sorry, I know this might seem like a trivial, silly detail to some, but to me this is incredibly tacky. Really, to pretend you are serving a tete de Cuvée Champagne and to try to pass off an English sparkling wine on unknowing customers? I love my wines and appreciate the fine taste of LPGS; I will know the difference and it would put a damper on my flight to drink an inferior wine pre-takeoff when BA makes a selling point of its LPGS.
I'll be sure to ask if the LPGS is open on the ground when I fly JFK-LHR in a few days; I'll keep you posted on what happens.
On another note, even for a first pour, I do think a proper F service should be to show the bottle and pour at one's seat. That's simply how it's done when one orders wine at any place of repute.