I don't have experience in BA F and have a JFK-LHR-JNB trip coming up and I'm wondering if I can get away with my Tumi 'Short Trip Expandable' as a carry-on?!? It's not a cabin approved size at 26.0" X 16.5" X 12.5" / 66cm X 42cm X 32. Trip includes 777, 789 and 380 - all in F. I certainly wouldn't expect to try this on a short-haul but curious if this is something I might be able to do for this trip.