Are buggy zone signage lawful or advisory signage.
I would regard it as advisory, based on this interpretation (Stagecoach's):
Subject to space being available and the discretion of the driver, we will carry small prams and unfolded buggies on low floor buses within the designated area but only when it is not required by a passenger in a wheelchair or approved mobility scooter (passengers in wheelchairs have absolute priority by law).