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One senior policeman told me that as a raw constable he was called to a massive ruck at a pub in Camberwell.A country boy, he'd never seen anything like it. He said, shaking with nerves, to the old sergeant with him "I suppose we have to go in, Sarge" and got the reply "No, stay here safe behind this wall. In a while they'll be only one left standing. Then go and nick him. He'll be no trouble because he's the winner and being nicked will prove it to everyone!" Dixon of Camberwell Green!
The police used to leave these pubs alone because a) fighting was part of life and b) they knew where all the tough blokes would be, all in one place; better that than have them starting trouble in all the pubs in the area, at random c) everyone knew the pubs and what to expect, and wouldn't go there in shocked innocence, and complain later.