You can report all criminal activity completely anonymously on CRIMESTOPPERS, the police anonymous reporting line. The national telephone nujmberif 0800 555 111.
This telephone number is manned by a team of individuals who liaise with the police in whatever area the crime or fraud is being conducted and they will follow it up. You will need to provide them with sufficient information regarding timing, venue, car registration number so that they can be sure of making a successful arrest.
And don't feel guilty about doing this. You may well be saving the life of an innocent adult or child.