It is advisable not to force your way into the car immediately but if the police cannot get there in time you then have to decide if you should take action.
If you decide to do this:
Make sure you tell the police what you intend to do.
It is important to tell the authorities why you are doing it, where you are, take photos of the dog and take the name of any witnesses.
It is within the law to commit damage to someone else's property if...
at the time of the act or acts alleged to constitute the offence you believed that the person or persons whom you believe to be entitled to consent to the destruction of or damage to the property in question . . . .would so consent to it if s/he . . . had known of the destruction or damage and its circumstances' (section 5(2)(a) Criminal Damage Act 1971).
Only do this if you are 100 per cent certain of the situation and are prepared to defend your actions in a court of law, should any action be taken against you.
If you would like to receive more guidance on information on cruelty please consult their specialist call line on 0300 1234 999.