On my BA flight to London recently, they offered me the ‘car seat’ for my 6 mth old baby. BA gives priority to babies in terms of age so the youngest babies get the box-like bassinets and the older babies get the car seats unless you are very unlucky and they have run out…. You don’t have to buy your child a seat until they are 2 years old.
I was a bit apprehensive at first as I wasn’t sure if she would sleep well in something that wasn’t flat. After an initial hiccup of me shaking the car seat with a scarf draped over my head & hers to create darkness she actually managed a good 7 hours.
On the way back I got the bassinet which she only just fit and again slept fairly well.
My advice for bassinets/car seats is to be pushy (in a nice way) from the moment you arrive at the airport, reminding the check in staff that you WILL be needing one for the flight. They might not know which type you will get but if you keep asking for it, they should provide one.