darren said:Ed W said:"reasonable force"
In most instances this would consist of the owner shouting "Stop breaking into my car" and the thief running off. Any more force than this in the circumstances would be an assault by the car owner.
I'm struggling to see how a sign stating this would help.
A common myth, that has little basis in fact. In reality, reasonable force can quite legally be used to stop a crime in progress.
https://www.cps.gov.uk/legal-guidance/self-defence-and-prevention-crime