It won’t be the police; normally the council will have a department set up for this who will also have an advice line, with recommended services. Just ring your local council as though you have a graffiti incident to see how they are handling the calls.
However be warned the councils are beginning to pick up on this themselves as a money earner as they have a responsibility to clean public areas anyway. Around here if you ask for graffiti removal, the council will send their own truck.