From the BA website
British Airways - Travelling with children
Car type safety seats
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 seat.
- The maximum dimensions of the seat must fit into an area of 45cm x 45cm (17.5ins x 17.5ins).
Please note: Two part car seats, where there is the seat and a base are not suitable for use on aircraft seats.