Car seat on BA flight

From their website
*What types of child restraint devices are allowed?
You may prefer to use your own car type safety seat, which must meet the following standards:
The seat must be designed to be secured by means of a normal aircraft single lap strap and face the same direction as the passenger seat on which it is positioned.
A purpose-designed children's car seat must have a 5-point restraining harness.
The car seat must not exceed the dimensions of the aircraft sear.
The maximum dimensions of the seat must fit into an area of 17.5x17.5 inches (45x45cm).
NOTE: Two part car seats, where there is the seat and a base are not suitable for use on aircraft seats.

You may use your own AmSafe System, which is an approved child restraint device, consisting of an additional belt and shoulder harness that encircles the aircraft seat back and attaches to the passenger lap belt.
It can be used for infants from 12 months to children of 3 years of age.
The AmSafe System is not suitable for use on First class or Club World sleeper seats, or World Traveler Plus seats.
It cannot be used in the same row as the emergency exit or the row in front or behind the emergency exit.
British Airways does not supply the AmSafe System, but you can bring your own with you.
*

You will need to check this seat into the hold as it will not be allowed on the aircraft seat.

You can’t. The seat belt won’t work. They are a burden onboard. Just check it in.

You won't be able to use it on board. You can check it into the hold