A cat in a cat stroller is as entitled to use a designated place on a bus as a child in a pram. There are two places on British buses for prams and if one of those places is taken up by a cat in a cat stroller then that leaves one place for a child in a pram. It is common sense because you cannot remove the cat from a cat stroller and fold up the stroller as this would be impractical and dangerous for the cat and indeed other passengers because you would have a cat roaming around the bus causing chaos. Or you might do and probably would.