Originally Posted by
fredandgingermad
Having another think about it, if a child is under the weight limit for it to be used forward facing in the car would that still apply on a plane?
Officially, yes, but it's better than just using the lap belt or being a lap child. And it enables you to have your seat handy for when you arrive to your destination.
US airlines allow you to rear-face in this scenario, but I'm not sure the same is the case for BA.