I just returned from Bangkok to UK and was offered a choice of a lie flat skycot, or a carseat style seat, which on a BA plane goes in the same place as the sky cot does. It's a metal frame with a padded seat that is shaped like a carseat. So baby is not lying flat, rather half sitting. The seat can sit up or recline. DS slept for 11 hours in it!! I didn't pay for a seat for him, but I was travelling business class. But the impression I got was that it was just another option other than the skycot, not something you'd pay child fare for, because it didn't take up a seat. Definitely ring and find out.
I don't think it would be available on Qantas because their skycots (in my experience) just fold down from the wall, there isn't something that clips into them like on BA flights.
Not sure about other airlines.
HTH