If flying with BA, beware of them saying the animal crate is too small even though a vet says it's big enough. A good friend of mine had to accommodate herself and two cats in the US for a surprise extra night when moving countries because BA rejected one of the crates despite a vet specifying them as acceptable, because you can't buy another animal crate at 19:30 in the evening in Boston airport. To give BA credit, they did transport and deliver the animals in good condition, but to give them a brickbat, they were a right bunch of jobsworths about it.
Live freight prices for two cats was about the same as one economy human SLF.