If I felt it was important to report I’d report; pretty sure the police are used to dealing with people coming forward who need anonymity; although in some cases the witness may be required to testify; but in other cases it may be the information leads to some actual police work followed by an arrest – so the information is only used to assist an operation.
I think as a rule a person’s safety has to always come first, but this should never come at the cost of someone else’s safety, this is where police protection comes in I guess. It’s v hard to write a response when it’s a hyperthetical and non specific q, but understand this is necessary here x