The 3-4-3 seating BA adopted on some 777 flights ex LGW in the early noughties were not technically BA flights. They were BA wetleased aircraft operating for AML. BA did add a BA code and the aircraft were BA aircraft with the normal BA product so this was rather confusing. AML itself was I believe a 50:50 joint venture between BA holidays and Thomas Cook. AML aircraft were operated by BA flight crew and dedicated AML Cabin Crew.

The Y configuration at that time was essentially designed to compete economically with the charter operators ex LGW flying similar longhaul leisure routes. Seating density was a key factor in making this work economically.

J service on these routes was typically very good as the crew made a real effort. I would think it highly unlikely that BA would adopt a 3-4-3 config more widely across mainline services given that the AML experiment was deemed a failure and it did not take very long to pull those services back to regular LGW mainline operations.