If your British Shorthair cat is displaying aggressive behavior, it is important to address the issue as soon as possible to prevent the behavior from becoming more entrenched. Here are some steps you can take:

1. Identify the triggers for the aggressive behavior. Does your cat become aggressive when they are playing, when they are being handled, or when they are confronted with certain stimuli? Understanding the triggers can help you to anticipate and prevent aggressive behavior.
2. Consult with a veterinarian or a feline behaviorist. Aggressive behavior can sometimes be a sign of an underlying medical issue or a result of stress or anxiety. A professional can help you to determine the cause of the aggression and recommend a course of treatment.
3. Use positive reinforcement to encourage good behavior. Reward your cat with treats, praise, and attention when they are displaying desirable behavior.
4. Provide your cat with plenty of mental and physical stimulation. Make sure your cat has plenty of toys and opportunities for play and exercise.
5. Avoid punishing your cat for aggressive behavior. Physical punishment or yelling can cause your cat to become even more anxious and aggressive.
6. Gradually expose your cat to the stimulus that triggers their aggression. With the help of a feline behaviorist, you can work to slowly desensitize your cat to the stimulus, using positive reinforcement to reward calm behavior.

It is important to be patient and consistent in addressing your cat’s aggressive behavior. It may take some time and effort, but with the right approach, you can help your cat to overcome their aggression and become a happier and more well-adjusted pet.