Excuse my use of AI, but this seems to explain it:

"Key Laws and Principles

Section 3(1) of the Criminal Law Act 1967 allows for the use of reasonable force in the prevention of crime.

Section 76 of the Criminal Justice and Immigration Act 2008 clarifies that the reasonableness of force is judged based on the circumstances as the person believed them to be, even if mistaken (unless due to voluntary intoxication). It also acknowledges that someone acting for a legitimate purpose may not be able to precisely measure the necessary action.

Common Law supports the right to a pre-emptive strike, meaning you don't have to wait to be physically attacked. This was confirmed in the case of Beckford v The Queen (1988), which stated that circumstances can justify a pre-emptive strike. "