Why did George and Jerome give up the idea of spending the night in the prison by fighting against a policeman ?

The men were afraid the policeman would respond to their assault by hitting them back and refusing to incarcerate them. 

George suggested walking back to Henley and assaulting a policeman, and so getting a night’s lodging in the station-house. But then there was the thought, “Suppose he only hits us back and refuses to lock us up!”