The British Shorthair's loud breathing sound may sound like nasal congestion because the British Shorthair cat's nasal cavity is relatively narrow, so the breathing sound will be louder. It may also be caused by upper respiratory tract infection, cold, throat or nasal cavity hyperplasia, etc. It depends on the cat. Are there any other illnesses? If it is a cold, you can feed the cat cold medicine. If it is caused by other diseases, it is best to take the cat to a pet hospital for examination and treatment.