answer=42
BA doesn't ban car seats it just has requirements of them.
They must have a integral shell type frame, (not bolted to the outside of the shell),
It needs to fit in the aircraft seat and not overhang it.
When the aircraft belt is fitted through the back of the seat , the buckle mustn't touch the seat frame.
The belts of the seat must be minimum 1" in width and be of a five point harness type, with a single latch quick release.
If it's the really fancy recliner type it can be reclined but must sit upright for takeoff landing and turbulence.
If it has a table this must be removed for take off and landing.
It sounds like a lot of rules but many good quality car seats do fit these rules.
They can't be fitted to exit rows or in the seat row forward and aft of the exit rows.
If the baby is less than 6months old it must sit on a lap for takeoff, landing and turbulence (don't ask me why I don't understand it either.) but it can use an approved car seat during the cruise.