re formula - i would take bottle filled with water plus formula powder in jar, then mix just before i went through security. take formula in bottles then add bottled water you request from the cabin crew...that's the best solution i could come up with. always take one extra empty bottle.
i travelled early december, was not allowed cartons through security, had to tip it into bottle and test.
be clear with airline staff, if they are 'looking after your formula' as they say they will, remind them it needs to be refrigerated...had early disaster.
BA flights sometimes carry SMA gold, not to be relied on though.
worth taking umbrella stroller as well as carrier if you have a long connection to let baby rest or even to aid you through the airport.
change of clothes for baby and a top for you in case of baby being sick. hygienic wipes, hand sanitiser and something to put on change tables in airport rooms/plane.
BA offer 'lap belts' for an infant, air canada cabin crew staff simply advised to hold infant on my shoulder, there was no restraint available. varies between airlines.
take familiar blanket esp if using bassinet cot long distance...airline magazines and safety card...it's amazing how interesting i had to make them sound. do your best, you can't get off halfway!! all my journeys have been easier than i had anticipated.
can't believe i've written so much...