Consent is a justificatory defence - the act is justified. There's no AR and the act isn't unlawful.
Consent is a defence to minor battery (e.g. pushing/shoving)
or ABH or GBH if the act is in the PUBLIC INTEREST -
Rough Horseplay - R v Jones / R v Aitken
Sport - IMPLIED consent to the risk of injury and level of contact - must be within the rules of the sport. R v Barnes - "instinctive...error in the heat of the game". Must consider type of sport…