If you were tripped an injured you'd either have to prove that the bar operators had been negligent or that the owner of the bag had been negligent (and that you could trace them. and that they had deep enough pockets to pay for the matter).
In an environment such as a bar the defence would no doubt try to argue contributory negligence, no doubt through alcohol consumption or some other route, that it would probably be a hell of a fight.
I think you should sue your own friends, TK. It is clear that they have engaged you to to carry out a service for them, so arguably you are an employee for that purpose, and thus they have responsibilities to ensure that your workplace is safe.
Were you given induction training, did they carry out even a cursory site check before asking you to enter the pub? Was a statutory poster displayed giving details of H&S? My blood boils when I hear of such negligence - sue them for every penny they've got!