I’m no lawyer and don’t know the exact legislation, but I believe in the UK you are allowed to use sufficient force in self-defence. That could include lethal force, if necessary. (Obviously, it’s down to the authorities and jury in the luxury of their own time to decide what constitutes sufficient force.) I understand you can only do this for personal defence, not for defence of any property. I.E. If you attack someone simply for the fact that they broke into your house or your car, when there was no immediate risk to anyone’s safety, then you can and almost certainly will be prosecuted for that.
English Common Law does allow for the use of force in protecting property as well as persons, or simply in preventing a crime. The concept was codified into statue law in the 1967 Criminal Justice Act:
A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large."
So if someone tries to steal your TV, or invades your home, your entitled to use a reasonable level of force to stop them. You’re just forbidden from engaging in any punitive violence - only what is necessary to prevent the crime or protect yourself, others around you and your property.