Sounds like common assault by battery.The least of assaults.Minor injuries (a graze or a black eye), although technically 'bodily harm', don't result in a charge of 'assault occasioning actual bodily harm' because a) you can elect jury trial for abh b) the CPS try to keep a sense of proportion and almost every assault would be abh otherwise.
If so, one punch,minor harm,out of character,had been innocent victim of violence, made mistake.....the mitigation stacks up. It's likely a fine but certainly no penalty to lose sleep over