If you are concerned that electoral fraud may have been committed, you should first speak to the Electoral Registration Officer, the relevant local Returning Officer or the CARO.
They may be able to explain whether or not electoral fraud has been committed, and can refer your concerns to the police if necessary. They can also provide you with the details of the police contact for the relevant police force so that you can report the allegation yourself.
If you have evidence that an electoral offence has been committed you should contact the police immediately, using the 101 non-emergency number unless there is a crime in progress in which case you should use the standard 999 emergency line.
Every police force has a dedicated Single Point of Contact Officer who will be able to provide advice to ensure that your allegations are properly investigated. You should be prepared to give them a statement and substantiate your allegation.
If you do not want to give a statement to the police, you can report your concerns anonymously on the Crimestoppers website or by calling Crimestoppers on 0800 555 111.