Yes, if you are genuinely worried then calling 999 and asking for the police is advisable. You will not be looked upon as a time waster and the report will be taken seriously. Explain the situation and let the operator know where you are and don’t hang-up unless advised to do so. The police should be able to rapidly confirm if it’s a valid vehicle and act accordingly if it isn't. The call handler will also be able to advise on a course of action.