Hey, UK folks, clue me in. In the United States, there's a widely mistaken belief that two people can simply decide to get in a fight, and it's all fine because they agreed to do it. In fact, indulging in such foolishness puts you at risk for a charge of assault, and the fact that the other party agreed to the fight is no defense. That's just the penalty before anyone even gets hurt, mind you. As soon as someone else does, you're either on the receiving end, or you're looking at additional criminal and civil penalties. Is the situation vastly different in the UK, so that people can simply go about challenging each other to fights, and as long as the other party says, "OK!" everybody gets to sleep at home that night?
Yup, that's a super-common urban legend in my state and profession. Nope, folks cant just choose to fight in public. Least not in my state. Off the top, a mutual public fight would be disturbing the peace. Probably get some assault charges as well, Just yelling threats at someone can get an assault charge. Self-defense claims get seriously muddled in mutual combat situations. One up conventions also get really messy when folks try to claim mutual combat.
Better solution. Dont fight.