The pub landlord has a liability here. From the description of the event the incident on stage was just the culmination of a longer incident which should have been obvious to staff and which should have been stopped by staff/landord before it got to the stage where some of the participants got onto the stage.

If the fighters were drunk all the more reason for staff to a) refuse to serve them anymore booze or b) to keep a close eye on their behaviour. I've worked in pubs, one of which was owned by my son, and, in any half decent establishment, that attitude would come as natural as breathing. There is a duty of care to ALL other occupants of the premises to ensure their safety and that means stopping trouble before it really gets started.