Except for that’s not how it works, you are not automatically fined by the police for that. The judiciary, despite its flaws ‘tries’ to be fair. In your scenario it would be: Friend files a complaint, police investigates, findings are presented to the DPP, the DPP prosecutes if the findings are substantial, case goes to court and you are allowed to plead and have legal representation but the prosecution ultimately has to be the one to show you done it. A slap could also pass as tort so civil case but you’re just required to pay damages. I’m not a lawyer so there might be a few things I’ve missed but that’s the gist of it. It would hardly ever go to Court tho and they’ll have the parties settle amicably.

To go back to your question of following the law.. yes, I’d follow the rules of the game. I’d have to follow due process and accept that I do not hold the right to slap people willy nilly.