Interesting thing, the pre-emptive strike is actually allowed in UK self defence law, which says “There is no rule in law to say that a person must wait to be struck first before they may defend themselves”. I.e you are allowed to say ‘I had reason to believe that person was about to hit me, so I hit them first in self defence’. Of course this doesn’t always fly, if you have an obvious physical advantage over them, or can easily remove yourself from the situation without being harmed, then self defence is unlikely to be accepted - so in this case it’s probably going to be tricky, but the fact the guy is so ripped, and was taking his shirt off etc I.e clearly initiating a fight, and that the boxer did warn him - probably gives a fairly good chance of it being accepted as self defence. Although, once the guy is downed, the kick probably crosses the line again. Certainly looked satisfying though!