How to inform the uk authorities of someone committing a crime?

Send a letter to your local Police station with any evidence you might have.... But I reckon even they wont do anything. Unfortunately if you want something to happen to a criminal in this country you have to get the evidence yourself to give the Police a reason for arresting them. The Police will only act if they know that they can get a conviction, which nowadays is very difficult due to the incompetence of the courts.