You can chase someone and strike them if you know they are fleeing after committing a crime. It is not illegal to do so. It is reasonable force in the UK, which has very strong reasonable force laws.
If, for example, I saw someone steal your mobile phone, wallet, handbag or whatever, it would be legal for me to chase them, force them to the ground and punch them in the head. That was the example stated by the home office and the police.
UK law allows for reasonable force in defence of self, others or property, to stop a crime in progress or to restrain a person you have personally witnessed committing a crime. Reasonable force is quite widely defined, essentially being whatever is needed to stop the person and seen in the context of the person using force (i.e. it is not a requirement to logically assess the minimum degree of force needed unless you could reasonably be expected to do so due to training and experience).
UK law is far stronger regarding defence than it is usually made out to be.
Sorry i was not fully aware of how the law works regarding this, but in this case it was not a legal option once they had left his house to chase them and attack them, or was it just the force of the attack?